exporters: prometheus: add exponential (native) histogram support (#6421)

Almost closes 5777.

Adding native (exponential) histogram support to the Prometheus
exporter. I tested it with a toy program, and the result looks good. I
added a unit test.

---------

Signed-off-by: Giedrius Statkevičius <giedrius.statkevicius@vinted.com>
Co-authored-by: Sam Xie <sam@samxie.me>

CHANGELOG.md

@@ -18,6 +18,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 ### Added
 
+- Add exponential histogram support in `go.opentelemetry.io/otel/exporters/prometheus`. (#6421)
 - The `go.opentelemetry.io/otel/semconv/v1.31.0` package.
   The package contains semantic conventions from the `v1.31.0` version of the OpenTelemetry Semantic Conventions.
   See the [migration documentation](./semconv/v1.31.0/MIGRATION.md) for information on how to upgrade from `go.opentelemetry.io/otel/semconv/v1.30.0`(#6479)

exporters/prometheus/exporter.go

@@ -8,6 +8,7 @@ import (
 	"encoding/hex"
 	"errors"
 	"fmt"
+	"math"
 	"slices"
 	"strings"
 	"sync"
@@ -241,6 +242,10 @@ func (c *collector) Collect(ch chan<- prometheus.Metric) {
 				addHistogramMetric(ch, v, m, name, kv)
 			case metricdata.Histogram[float64]:
 				addHistogramMetric(ch, v, m, name, kv)
+			case metricdata.ExponentialHistogram[int64]:
+				addExponentialHistogramMetric(ch, v, m, name, kv)
+			case metricdata.ExponentialHistogram[float64]:
+				addExponentialHistogramMetric(ch, v, m, name, kv)
 			case metricdata.Sum[int64]:
 				addSumMetric(ch, v, m, name, kv)
 			case metricdata.Sum[float64]:
@@ -254,6 +259,60 @@ func (c *collector) Collect(ch chan<- prometheus.Metric) {
 	}
 }
 
+func addExponentialHistogramMetric[N int64 | float64](
+	ch chan<- prometheus.Metric,
+	histogram metricdata.ExponentialHistogram[N],
+	m metricdata.Metrics,
+	name string,
+	kv keyVals,
+) {
+	for _, dp := range histogram.DataPoints {
+		keys, values := getAttrs(dp.Attributes)
+		keys = append(keys, kv.keys...)
+		values = append(values, kv.vals...)
+
+		desc := prometheus.NewDesc(name, m.Description, keys, nil)
+
+		// From spec: note that Prometheus Native Histograms buckets are indexed by upper boundary while Exponential Histograms are indexed by lower boundary, the result being that the Offset fields are different-by-one.
+		positiveBuckets := make(map[int]int64)
+		for i, c := range dp.PositiveBucket.Counts {
+			if c > math.MaxInt64 {
+				otel.Handle(fmt.Errorf("positive count %d is too large to be represented as int64", c))
+				continue
+			}
+			positiveBuckets[int(dp.PositiveBucket.Offset)+i+1] = int64(c) // nolint: gosec  // Size check above.
+		}
+
+		negativeBuckets := make(map[int]int64)
+		for i, c := range dp.NegativeBucket.Counts {
+			if c > math.MaxInt64 {
+				otel.Handle(fmt.Errorf("negative count %d is too large to be represented as int64", c))
+				continue
+			}
+			negativeBuckets[int(dp.NegativeBucket.Offset)+i+1] = int64(c) // nolint: gosec  // Size check above.
+		}
+
+		m, err := prometheus.NewConstNativeHistogram(
+			desc,
+			dp.Count,
+			float64(dp.Sum),
+			positiveBuckets,
+			negativeBuckets,
+			dp.ZeroCount,
+			dp.Scale,
+			dp.ZeroThreshold,
+			dp.StartTime,
+			values...)
+		if err != nil {
+			otel.Handle(err)
+			continue
+		}
+
+		// TODO(GiedriusS): add exemplars here after https://github.com/prometheus/client_golang/pull/1654#pullrequestreview-2434669425 is done.
+		ch <- m
+	}
+}
+
 func addHistogramMetric[N int64 | float64](
 	ch chan<- prometheus.Metric,
 	histogram metricdata.Histogram[N],
@@ -468,6 +527,8 @@ func convertsToUnderscore(b rune) bool {
 
 func (c *collector) metricType(m metricdata.Metrics) *dto.MetricType {
 	switch v := m.Data.(type) {
+	case metricdata.ExponentialHistogram[int64], metricdata.ExponentialHistogram[float64]:
+		return dto.MetricType_HISTOGRAM.Enum()
 	case metricdata.Histogram[int64], metricdata.Histogram[float64]:
 		return dto.MetricType_HISTOGRAM.Enum()
 	case metricdata.Sum[float64]:

exporters/prometheus/exporter_test.go

@@ -36,6 +36,7 @@ func TestPrometheusExporter(t *testing.T) {
 		options             []Option
 		expectedFile        string
 		disableUTF8         bool
+		checkMetricFamilies func(t testing.TB, dtos []*dto.MetricFamily)
 	}{
 		{
 			name:         "counter",
@@ -172,6 +173,50 @@ func TestPrometheusExporter(t *testing.T) {
 				gauge.Add(ctx, -.25, opt)
 			},
 		},
+		{
+			name:         "exponential histogram",
+			expectedFile: "testdata/exponential_histogram.txt",
+			checkMetricFamilies: func(t testing.TB, mfs []*dto.MetricFamily) {
+				var hist *dto.MetricFamily
+
+				for _, mf := range mfs {
+					if *mf.Name == `exponential_histogram_baz_bytes` {
+						hist = mf
+						break
+					}
+				}
+
+				if hist == nil {
+					t.Fatal("expected to find histogram")
+				}
+
+				m := hist.GetMetric()[0].Histogram
+
+				require.Equal(t, 236.0, *m.SampleSum)
+				require.Equal(t, uint64(4), *m.SampleCount)
+				require.Equal(t, []int64{1, -1, 1, -1, 2}, m.PositiveDelta)
+				require.Equal(t, uint32(5), *m.PositiveSpan[0].Length)
+				require.Equal(t, int32(3), *m.PositiveSpan[0].Offset)
+			},
+			recordMetrics: func(ctx context.Context, meter otelmetric.Meter) {
+				// NOTE(GiedriusS): there is no text format for exponential (native)
+				// histograms so we don't expect any output.
+				opt := otelmetric.WithAttributes(
+					attribute.Key("A").String("B"),
+					attribute.Key("C").String("D"),
+				)
+				histogram, err := meter.Float64Histogram(
+					"exponential_histogram_baz",
+					otelmetric.WithDescription("a very nice histogram"),
+					otelmetric.WithUnit("By"),
+				)
+				require.NoError(t, err)
+				histogram.Record(ctx, 23, opt)
+				histogram.Record(ctx, 7, opt)
+				histogram.Record(ctx, 101, opt)
+				histogram.Record(ctx, 105, opt)
+			},
+		},
 		{
 			name:         "histogram",
 			expectedFile: "testdata/histogram.txt",
@@ -517,7 +562,14 @@ func TestPrometheusExporter(t *testing.T) {
 					metric.Stream{Aggregation: metric.AggregationExplicitBucketHistogram{
 						Boundaries: []float64{0, 5, 10, 25, 50, 75, 100, 250, 500, 1000},
 					}},
-				)),
+				),
+					metric.NewView(
+						metric.Instrument{Name: "exponential_histogram_*"},
+						metric.Stream{Aggregation: metric.AggregationBase2ExponentialHistogram{
+							MaxSize: 10,
+						}},
+					),
+				),
 			)
 			meter := provider.Meter(
 				"testmeter",
@@ -533,6 +585,15 @@ func TestPrometheusExporter(t *testing.T) {
 
 			err = testutil.GatherAndCompare(registry, file)
 			require.NoError(t, err)
+
+			if tc.checkMetricFamilies == nil {
+				return
+			}
+
+			mfs, err := registry.Gather()
+			require.NoError(t, err)
+
+			tc.checkMetricFamilies(t, mfs)
 		})
 	}
 }

exporters/prometheus/testdata/exponential_histogram.txt

@@ -0,0 +1,11 @@
+# HELP exponential_histogram_baz_bytes a very nice histogram
+# TYPE exponential_histogram_baz_bytes histogram
+exponential_histogram_baz_bytes_bucket{A="B",C="D",otel_scope_name="testmeter",otel_scope_version="v0.1.0",le="+Inf"} 4
+exponential_histogram_baz_bytes_sum{A="B",C="D",otel_scope_name="testmeter",otel_scope_version="v0.1.0"} 236
+exponential_histogram_baz_bytes_count{A="B",C="D",otel_scope_name="testmeter",otel_scope_version="v0.1.0"} 4
+# HELP otel_scope_info Instrumentation Scope metadata
+# TYPE otel_scope_info gauge
+otel_scope_info{fizz="buzz",otel_scope_name="testmeter",otel_scope_version="v0.1.0"} 1
+# HELP target_info Target metadata
+# TYPE target_info gauge
+target_info{"service.name"="prometheus_test","telemetry.sdk.language"="go","telemetry.sdk.name"="opentelemetry","telemetry.sdk.version"="latest"} 1