5 files changed, 589 insertions, 0 deletions
diff --git a/microprom/handler.go b/microprom/handler.go
new file mode 100644
index 0000000..a505df3
--- /dev/null
+++ b/microprom/handler.go
@@ -0,0 +1,171 @@
+// SPDX-FileCopyrightText: 2026 Stefan Majewsky <majewsky@gmx.net>
+// SPDX-License-Identifier: Apache-2.0
+
+package microprom
+
+import (
+	"bufio"
+	"context"
+	"fmt"
+	"io"
+	"maps"
+	"net/http"
+	"slices"
+	"strings"
+
+	"go.xyrillian.de/gg/internal/accept"
+)
+
+// Handler is an [http.Handler] rendering metrics in Prometheus exposition formats.
+//
+// If SortOutput is false:
+//   - Metric families will be printed in undefined order.
+//   - Metrics within the same family will be printed in the order in which they were added.
+//   - This behavior is the default because it is more efficient.
+//
+// If SortOutput is true:
+//   - Metric families will be sorted by name.
+//   - Metrics within the same family will be sorted by Labels.
+//   - This behavior may be useful in tests because it produces deterministic output.
+//
+// When asserting on metrics in tests, it may be useful to set SortOutput equal to testing.Testing().
+type Handler struct {
+	// The set of metric families for which this handler can report metrics.
+	Families map[MetricFamilyName]MetricFamilyInfo
+	// This function will be called for each request to the handler.
+	// The implementation shall provide metrics by calling [MetricSet.Add].
+	Collect func(context.Context, *MetricSet) error
+
+	// See documentation on type for details.
+	SortOutput bool
+}
+
+var _ http.Handler = Handler{}
+
+// ServeHTTP implements the [http.Handler] interface.
+func (h Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	acceptedFormat, ok := accept.ParseHeader(r.Header["Accept"]).Negotiate(
+		// The way that Prometheus handles `Accept` is insane.
+		// They put a billion parameters in there, with `escaping=` possibly depending on server configuration.
+		// (I have not read enough of the Prometheus source code to be sure.)
+		// The easiest way for us is to negotiate for all possible combinations.
+		//
+		// Note that it is fine for the client to request less specific formats, e.g. just "application/openmetrics-text",
+		// in which case the first match will be used.
+		// Each set of similar choices has `escaping=underscores` on top each time because that's the default escaping scheme in promhttp.
+		"text/plain; version=0.0.4; charset=utf-8; escaping=underscores",
+		"text/plain; version=0.0.4; charset=utf-8; escaping=allow-utf-8",
+		"text/plain; version=0.0.4; charset=utf-8; escaping=dots",
+		"text/plain; version=0.0.4; charset=utf-8; escaping=values",
+		"application/openmetrics-text; version=1.0.0; charset=utf-8; escaping=underscores",
+		"application/openmetrics-text; version=1.0.0; charset=utf-8; escaping=allow-utf-8",
+		"application/openmetrics-text; version=1.0.0; charset=utf-8; escaping=dots",
+		"application/openmetrics-text; version=1.0.0; charset=utf-8; escaping=values",
+	).Unpack()
+	if !ok {
+		http.Error(w, "supported formats are text/plain and application/openmetrics-text", http.StatusNotAcceptable)
+		return
+	}
+
+	w.Header().Set("Content-Type", acceptedFormat)
+	syntax := SyntaxPrometheusLegacy
+	if strings.HasPrefix(acceptedFormat, "application/openmetrics-text; version=1.0.0;") {
+		syntax = SyntaxOpenMetricsV1
+	}
+
+	ms := NewMetricSet(syntax, h.Families)
+	err := h.Collect(r.Context(), ms)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+
+	w.WriteHeader(http.StatusOK)
+	bw := bufio.NewWriter(w)
+	if h.SortOutput {
+		for _, familyName := range slices.Sorted(maps.Keys(h.Families)) {
+			h.printMetricFamily(bw, syntax, familyName, h.Families[familyName], ms.metrics[familyName])
+		}
+	} else {
+		for familyName, familyInfo := range h.Families {
+			h.printMetricFamily(bw, syntax, familyName, familyInfo, ms.metrics[familyName])
+		}
+	}
+
+	if syntax != SyntaxPrometheusLegacy {
+		fmt.Fprint(bw, "# EOF\n")
+	}
+	err = bw.Flush()
+	if err != nil {
+		// We do not have a way to log this because we do not know what log library the application uses,
+		// and I also do not want to add a dependency injection slot to type Handler for this one extremely unlikely codepath.
+		// So instead, we're just going to wreck the response body and hope that Prometheus
+		// or whatever else receives this logs this as a syntax error or something.
+		fmt.Fprintf(w, "flush error: %s\n", err.Error())
+	}
+}
+
+func (h Handler) printMetricFamily(w io.Writer, syntax Syntax, familyName MetricFamilyName, info MetricFamilyInfo, metrics []metric) {
+	if len(metrics) == 0 {
+		return
+	}
+
+	var metricName string
+	switch info.Type {
+	case MetricTypeGauge:
+		metricName = string(familyName)
+	case MetricTypeCounter:
+		metricName = string(familyName) + "_total"
+	case MetricTypeInfo:
+		metricName = string(familyName) + "_info"
+	default:
+		panic("unreachable") // NewMetricSet() should have rejected unknown MetricType values
+	}
+
+	if syntax == SyntaxPrometheusLegacy {
+		// Prometheus Text Format does not distinguish between metric names and metric family names
+		familyName = MetricFamilyName(metricName)
+	}
+
+	fmt.Fprintf(w, "# HELP %s %s\n# TYPE %s %s\n", familyName, info.Help, familyName, metricTypeNames[info.Type])
+
+	if h.SortOutput {
+		slices.SortFunc(metrics, func(lhs, rhs metric) int {
+			return strings.Compare(string(lhs.labels), string(rhs.labels))
+		})
+	}
+	for _, m := range metrics {
+		if m.labels == "" {
+			fmt.Fprintf(w, "%s ", metricName)
+		} else {
+			fmt.Fprintf(w, "%s{%s} ", metricName, m.labels)
+		}
+		if syntax == SyntaxPrometheusLegacy {
+			fmt.Fprintf(w, "%g\n", m.value)
+		} else {
+			// TODO: ugly
+			fi := floatInspector{inner: w}
+			fmt.Fprintf(&fi, "%g", m.value)
+			if fi.clearlyFloat {
+				fmt.Fprintf(w, "\n")
+			} else {
+				fmt.Fprintf(w, ".0\n")
+			}
+		}
+	}
+}
+
+type floatInspector struct {
+	inner        io.Writer
+	clearlyFloat bool
+}
+
+func (fi *floatInspector) Write(buf []byte) (int, error) {
+	if slices.Contains(buf, '.') {
+		fi.clearlyFloat = true
+	}
+	if slices.Contains(buf, 'e') {
+		fi.clearlyFloat = true
+	}
+	return fi.inner.Write(buf)
+}
diff --git a/microprom/handler_test.go b/microprom/handler_test.go
new file mode 100644
index 0000000..7278945
--- /dev/null
+++ b/microprom/handler_test.go
@@ -0,0 +1,142 @@
+// SPDX-FileCopyrightText: 2026 Stefan Majewsky <majewsky@gmx.net>
+// SPDX-License-Identifier: Apache-2.0
+
+package microprom_test
+
+import (
+	"context"
+	"errors"
+	"io"
+	"maps"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"go.xyrillian.de/gg/assert"
+	"go.xyrillian.de/gg/microprom"
+)
+
+func TestHandlerBasic(t *testing.T) {
+	// NOTE: Most happy path coverage is in `./testing/microprom`.
+	//       This only covers the SortOutput = false case.
+
+	h := microprom.Handler{
+		Families: map[microprom.MetricFamilyName]microprom.MetricFamilyInfo{
+			"process": {
+				Type: microprom.MetricTypeInfo,
+				Help: "Information about this process.",
+			},
+			"foo": {
+				Type: microprom.MetricTypeGauge,
+				Help: "This metric family will not have any collected metrics and thus go unreported.",
+			},
+		},
+		Collect: func(ctx context.Context, ms *microprom.MetricSet) error {
+			names := microprom.NewLabelNames("version")
+			labels := ms.FormatLabels(names, "1.2.3")
+			ms.Add("process", labels, 1.0)
+			return nil
+		},
+	}
+
+	// test normal behavior
+	status, body, headers := getMetrics(t, h, nil)
+	assert.Equal(t, status, http.StatusOK)
+	assert.Equal(t, headers, http.Header{
+		"Content-Type": {"text/plain; version=0.0.4; charset=utf-8; escaping=underscores"},
+	})
+	assert.Equal(t, body, strings.TrimSpace(`
+# HELP process_info Information about this process.
+# TYPE process_info info
+process_info{version="1.2.3"} 1
+	`)+"\n")
+}
+
+func TestHandlerErrors(t *testing.T) {
+	h := microprom.Handler{
+		Families: map[microprom.MetricFamilyName]microprom.MetricFamilyInfo{
+			"process": {
+				Type: microprom.MetricTypeInfo,
+				Help: "Information about this process.",
+			},
+		},
+		Collect: func(ctx context.Context, ms *microprom.MetricSet) error {
+			return errors.New("kaboom")
+		},
+	}
+
+	// test unacceptable content negotiation
+	status, body, headers := getMetrics(t, h, http.Header{"Accept": {"application/json"}})
+	assert.Equal(t, status, http.StatusNotAcceptable)
+	assert.Equal(t, headers.Get("Content-Type"), "text/plain; charset=utf-8")
+	assert.Equal(t, body, "supported formats are text/plain and application/openmetrics-text\n")
+
+	// test error during h.Collect()
+	status, body, headers = getMetrics(t, h, nil)
+	assert.Equal(t, status, http.StatusInternalServerError)
+	assert.Equal(t, headers.Get("Content-Type"), "text/plain; charset=utf-8")
+	assert.Equal(t, body, "kaboom\n")
+
+	// test panic from invalid metric family name
+	h.Families["what is this?"] = microprom.MetricFamilyInfo{
+		Type: microprom.MetricTypeGauge,
+		Help: "invalid metric family name",
+	}
+	msg := assert.PanicsWith[string](t, func() { getMetrics(t, h, nil) })
+	assert.Equal(t, msg, `in family "what is this?": invalid family name (does not match /^[a-zA-Z_:][a-zA-Z0-9_:]*$/)`)
+	delete(h.Families, "what is this?")
+
+	// test panic from invalid metric type
+	h.Families["invalid"] = microprom.MetricFamilyInfo{
+		Type: 100,
+		Help: "invalid metric type",
+	}
+	msg = assert.PanicsWith[string](t, func() { getMetrics(t, h, nil) })
+	assert.Equal(t, msg, `in family "invalid": invalid value for microprom.MetricType: 100`)
+	delete(h.Families, "invalid")
+
+	// test panic from invalid label name
+	h.Collect = func(ctx context.Context, ms *microprom.MetricSet) error {
+		names := microprom.NewLabelNames("app:version")
+		labels := ms.FormatLabels(names, "1.2.3")
+		ms.Add("process", labels, 1.0)
+		return nil
+	}
+	msg = assert.PanicsWith[string](t, func() { getMetrics(t, h, nil) })
+	assert.Equal(t, msg, `invalid label name: "app:version"`)
+
+	// test panic from wrong number of label values
+	h.Collect = func(ctx context.Context, ms *microprom.MetricSet) error {
+		names := microprom.NewLabelNames("version", "build_date")
+		labels := ms.FormatLabels(names, "1.2.3") // forgot build_date
+		ms.Add("process", labels, 1.0)
+		return nil
+	}
+	msg = assert.PanicsWith[string](t, func() { getMetrics(t, h, nil) })
+	assert.Equal(t, msg, `expected 2 label values, but got 1`)
+
+	// test panic from using an undeclared metric family
+	h.Collect = func(ctx context.Context, ms *microprom.MetricSet) error {
+		ms.Add("invalid", "", 1.0)
+		return nil
+	}
+	msg = assert.PanicsWith[string](t, func() { getMetrics(t, h, nil) })
+	assert.Equal(t, msg, `no such family: invalid`)
+}
+
+func getMetrics(t *testing.T, h http.Handler, requestHeaders http.Header) (status int, responseBody string, responseHeaders http.Header) {
+	t.Helper()
+	r := httptest.NewRequestWithContext(t.Context(), http.MethodGet, "/metrics", nil)
+	maps.Copy(r.Header, requestHeaders)
+
+	w := httptest.NewRecorder()
+	h.ServeHTTP(w, r)
+	resp := w.Result()
+
+	buf, err := io.ReadAll(resp.Body)
+	if err != nil {
+		t.Fatal(err.Error())
+	}
+	return resp.StatusCode, string(buf), resp.Header
+}
diff --git a/microprom/labels.go b/microprom/labels.go
new file mode 100644
index 0000000..5b56c9c
--- /dev/null
+++ b/microprom/labels.go
@@ -0,0 +1,94 @@
+// SPDX-FileCopyrightText: 2026 Stefan Majewsky <majewsky@gmx.net>
+// SPDX-License-Identifier: Apache-2.0
+
+package microprom
+
+import (
+	"fmt"
+	"strings"
+)
+
+// Labels holds a label set, formatted according to the text protocol of [OpenMetrics 1.0].
+// Instances are constructed through [LabelNames.Format].
+//
+// [OpenMetrics 1.0]: https://prometheus.io/docs/specs/om/open_metrics_spec/
+type Labels string
+
+// LabelNames holds a set of label names.
+type LabelNames struct {
+	// NOTE: This is an opaque struct because, when adding support for OpenMetrics 2.0,
+	// it will be useful to precompute escaped forms for these names where necessary.
+	//
+	// Another interesting addition might be alphabetical sorting of labels, where we
+	// would need to remember the sort order because we need to apply it to the values
+	// slice during Format().
+	names []string
+}
+
+// NewLabelNames constructs a LabelNames instance.
+//
+// Per the [OpenMetrics 1.0] spec, label names must match the following regular expression:
+//
+//	[a-zA-Z_][a-zA-Z0-9_]*
+//
+// [OpenMetrics 1.0]: https://prometheus.io/docs/specs/om/open_metrics_spec/
+func NewLabelNames(names ...string) LabelNames {
+	for _, name := range names {
+		if !labelNameRx.MatchString(name) {
+			panic(fmt.Sprintf("invalid label name: %q", name))
+		}
+	}
+	return LabelNames{names}
+}
+
+// FormatLabels serializes a Prometheus labelset into the string format used in Prometheus text expositions.
+// For example:
+//
+//	// once, e.g. during func init()
+//	var names = microprom.NewLabelNames("foo", "hello")
+//
+//	// during microprom.Handler.Collect()
+//	labels := ms.FormatLabels(names, "bar", "world")
+//	assert.Equal(t, labels, `foo="bar",hello="world"`)
+func (ms *MetricSet) FormatLabels(n LabelNames, values ...string) Labels {
+	// NOTE on API structure: This is not part of ms.Add() to allow reusing label sets for multiple metrics.
+
+	if len(n.names) != len(values) {
+		panic(fmt.Sprintf("expected %d label values, but got %d", len(n.names), len(values)))
+	}
+	if len(n.names) == 0 {
+		return ""
+	}
+
+	// estimate the perfect number of bytes for the result string to avoid reallocations
+	capacity := len(n.names) - 1 // number of "," between pairs
+	needsEscaping := make([]bool, len(n.names))
+	for idx, value := range values {
+		// base length for an encoding in the form `label="value"`
+		capacity += len(n.names[idx]) + len(value) + 3
+		// some characters within `value` need escaping (TODO: this could be optimized to only iterate through `value` once)
+		toEscape := strings.Count(value, "\n") + strings.Count(value, "\"") + strings.Count(value, "\\")
+		needsEscaping[idx] = toEscape > 0
+		capacity += toEscape
+	}
+
+	var b strings.Builder
+	b.Grow(capacity)
+	for idx, value := range values {
+		if idx > 0 {
+			_ = b.WriteByte(',')
+		}
+		_, _ = b.WriteString(n.names[idx])
+		_ = b.WriteByte('=')
+		_ = b.WriteByte('"')
+		if needsEscaping[idx] {
+			// TODO: this could be optimized, but since this branch is unlikely in practice, I did not bother yet
+			value = strings.ReplaceAll(value, "\\", "\\\\")
+			value = strings.ReplaceAll(value, "\"", "\\\"")
+			value = strings.ReplaceAll(value, "\n", "\\n")
+		}
+		_, _ = b.WriteString(value)
+		_ = b.WriteByte('"')
+	}
+	return Labels(b.String())
+}
diff --git a/microprom/labels_test.go b/microprom/labels_test.go
new file mode 100644
index 0000000..0414be8
--- /dev/null
+++ b/microprom/labels_test.go
@@ -0,0 +1,24 @@
+// SPDX-FileCopyrightText: 2026 Stefan Majewsky <majewsky@gmx.net>
+// SPDX-License-Identifier: Apache-2.0
+
+package microprom_test
+
+import (
+	"testing"
+
+	"go.xyrillian.de/gg/assert"
+	"go.xyrillian.de/gg/microprom"
+)
+
+func TestFormatLabels(t *testing.T) {
+	ms := microprom.NewMetricSet(microprom.SyntaxOpenMetricsV1, nil)
+
+	// the basic example from the documentation
+	names := microprom.NewLabelNames("foo", "hello")
+	labels := ms.FormatLabels(names, "bar", "world")
+	assert.Equal(t, labels, `foo="bar",hello="world"`)
+
+	// test escaping label values
+	labels = ms.FormatLabels(names, "bar\\\n\\bar", `"universe\world"`)
+	assert.Equal(t, labels, `foo="bar\\\n\\bar",hello="\"universe\\world\""`)
+}
diff --git a/microprom/microprom.go b/microprom/microprom.go
new file mode 100644
index 0000000..4055c8b
--- /dev/null
+++ b/microprom/microprom.go
@@ -0,0 +1,158 @@
+// SPDX-FileCopyrightText: 2026 Stefan Majewsky <majewsky@gmx.net>
+// SPDX-License-Identifier: Apache-2.0
+
+// Package microprom is a minimal alternative implementation of [promhttp],
+// intended for very specific situations where the design choices of [prometheus/client_golang]
+// cause scaling problems:
+//   - metric families with very high cardinality,
+//   - that may have lots of label dimensions,
+//   - and which do not need to be held in memory, but can instead easily be generated at scrape time (e.g. from a database query).
+//
+// In this specific circumstance, the internal structure of [prometheus/client_golang]
+// leads to abnormally high memory fragmentation and a spiky memory usage pattern overall.
+// Implementing the same metrics endpoint with microprom will lead to a
+// more stable memory consumption with less intense spikes during scrapes,
+// at the cost of slightly more CPU time cost and GC pressure.
+//
+// A microprom handler produces output in the [Prometheus exposition format], matching the output of promhttp exactly;
+// thus it can be scraped by Prometheus or any other OpenTelemetry-compatible metrics collector.
+// However, because of the highly specialized focus on high-cardinality database metrics,
+// significant parts of the OTLP Stream Model (e.g. summaries, histograms, exemplars) are not implemented.
+// The only supported metric types are gauges, counters and info metrics.
+//
+// # How to use
+//
+// To get started with microprom, declare your metric families in a [Metadata] instance,
+// and then call [Metadata.Handler] to obtain a handler for your "GET /metrics" endpoint.
+//
+// [promhttp]: https://pkg.go.dev/github.com/prometheus/client_golang/prometheus/promhttp
+// [prometheus/client_golang]: https://pkg.go.dev/github.com/prometheus/client_golang
+// [Prometheus exposition format]: https://prometheus.io/docs/instrumenting/exposition_formats/
+package microprom
+
+import (
+	"fmt"
+	"regexp"
+)
+
+// MetricFamilyInfo appears in type [HandlerInfo].
+type MetricFamilyInfo struct {
+	Type MetricType
+	Help string
+}
+
+var (
+	labelNameRx        = regexp.MustCompile(`^[a-zA-Z_][a-zA-Z0-9_]*$`)
+	metricFamilyNameRx = regexp.MustCompile(`^[a-zA-Z_:][a-zA-Z0-9_:]*$`)
+)
+
+func (i MetricFamilyInfo) validate(name MetricFamilyName) error {
+	if !metricFamilyNameRx.MatchString(string(name)) {
+		return fmt.Errorf("in family %q: invalid family name (does not match /%s/)", name, metricFamilyNameRx.String())
+	}
+	if i.Type >= MetricType(len(metricTypeSuffixes)) {
+		return fmt.Errorf("in family %q: invalid value for microprom.MetricType: %d", name, i.Type)
+	}
+	return nil
+}
+
+// MetricFamilyName is the name of a metric family.
+//
+// Per the [OpenMetrics 1.0] spec, metric family names must match the following regular expression:
+//
+//	^[a-zA-Z_:][a-zA-Z0-9_:]*$
+//
+// Package microprom does not implement escaping at the moment;
+// metric family names not matching this pattern are invalid and will cause a panic.
+// This restriction may be lifted in a future version.
+//
+// [OpenMetrics 1.0]: https://prometheus.io/docs/specs/om/open_metrics_spec/
+type MetricFamilyName string
+
+// MetricType is a enum. It appears in type [MetricFamilyInfo].
+//
+// As documented on the individual values below,
+// the choice of metric type determines how [MetricSet.Add] derives the metric name.
+type MetricType uint
+
+const (
+	// MetricTypeGauge is used for metrics that are current measurements,
+	// where the absolute value is of interest to a user.
+	//
+	// For this metric type, the metric name is the same as the metric family name.
+	MetricTypeGauge MetricType = iota
+
+	// MetricTypeCounter is used for counting discrete events,
+	// where the rate of increase over time is of interest to a user.
+	//
+	// For this metric type, the metric name is formed by appending "_total" to the metric family name.
+	MetricTypeCounter
+
+	// MetricTypeInfo is used for info metrics,
+	// which only expose textual information in their labels.
+	//
+	// For this metric type, the metric name is formed by appending "_info" to the metric family name.
+	MetricTypeInfo
+)
+
+var (
+	metricTypeNames    = []string{"gauge", "counter", "info"}
+	metricTypeSuffixes = []string{"", "_total", "_info"}
+)
+
+// MetricSet holds a set of metrics.
+type MetricSet struct {
+	syntax  Syntax
+	metrics map[MetricFamilyName][]metric
+}
+
+type metric struct {
+	labels Labels
+	value  float64
+}
+
+// NewMetricSet constructs an initially empty [MetricSet] that accepts metrics for the given metric families.
+func NewMetricSet(syntax Syntax, families map[MetricFamilyName]MetricFamilyInfo) *MetricSet {
+	if syntax > SyntaxOpenMetricsV1 {
+		panic(fmt.Sprintf("unknown value for Syntax: %d", syntax))
+	}
+	m := make(map[MetricFamilyName][]metric, len(families))
+	for name, family := range families {
+		err := family.validate(name)
+		if err != nil {
+			// this is fine to panic because it will only blow up in case of gross API misuse
+			panic(err.Error())
+		}
+		m[name] = nil
+	}
+	return &MetricSet{syntax, m}
+}
+
+// Add adds a metric to the MetricSet.
+//
+// The name must be of a metric family that was declared during [NewMetricSet], otherwise Add will panic.
+// The metric name will be derived according to the rules documented on the respective [MetricType].
+func (ms *MetricSet) Add(name MetricFamilyName, labels Labels, value float64) {
+	_, ok := ms.metrics[name]
+	if !ok {
+		panic("no such family: " + string(name))
+	}
+	ms.metrics[name] = append(ms.metrics[name], metric{labels, value})
+}
+
+// Syntax is an enum, defining which exposition format will be used by [MetricSet].
+//
+//   - SyntaxPrometheusLegacy corresponds to the [Prometheus Text Format].
+//   - SyntaxOpenMetricsV1 corresponds to the [OpenMetrics 1.0] text format
+//   - Additional formats may be added in the future (e.g. OpenMetrics 2.0, once it is stabilized).
+//
+// [Prometheus Text Format]: https://prometheus.io/docs/instrumenting/exposition_formats/
+// [OpenMetrics 1.0]: https://prometheus.io/docs/specs/om/open_metrics_spec/
+type Syntax uint
+
+const (
+	// SyntaxPrometheusLegacy corresponds to the Prometheus text format (currently version 0.0.4).
+	SyntaxPrometheusLegacy Syntax = iota
+	// SyntaxOpenMetricsV1 corresponds to the OpenMetrics 1.0 text format.
+	SyntaxOpenMetricsV1
+)