Support exponential histograms in the prometheus bridge

CHANGELOG.md

@@ -16,6 +16,7 @@ The next release will require at least [Go 1.21].
 - Add the new `go.opentelemetry.io/contrib/instrgen` package to provide auto-generated source code instrumentation. (#3068, #3108)
 - Support [Go 1.22]. (#5082)
 - Add support for Summary metrics to `go.opentelemetry.io/contrib/bridges/prometheus`. (#5089)
+- Add support for Exponential (native) Histograms in `go.opentelemetry.io/contrib/bridges/prometheus`. (#5093)
 
 ### Removed
 

bridges/prometheus/doc.go

@@ -18,9 +18,11 @@
 // with the OpenTelemetry SDK. This enables prometheus instrumentation libraries
 // to be used with OpenTelemetry exporters, including OTLP.
 //
-// Limitations:
-//   - Prometheus histograms are translated to OpenTelemetry fixed-bucket
-//     histograms, rather than exponential histograms.
+// Prometheus histograms are translated to OpenTelemetry exponential histograms
+// when native histograms are enabled in the Prometheus client. To enable
+// Prometheus native histograms, set the (currently experimental) NativeHistogram...
+// options of the prometheus [HistogramOpts] when creating prometheus histograms.
 //
 // [Prometheus Golang client library]: https://github.com/prometheus/client_golang
+// [HistogramOpts]: https://pkg.go.dev/github.com/prometheus/client_golang/prometheus#HistogramOpts
 package prometheus // import "go.opentelemetry.io/contrib/bridges/prometheus"

bridges/prometheus/producer.go

@@ -90,6 +90,10 @@
 	var errs multierr
 	otelMetrics := make([]metricdata.Metrics, 0)
 	for _, pm := range promMetrics {
+		if len(pm.GetMetric()) == 0 {
+			// This shouldn't ever happen
+			continue
+		}
 		newMetric := metricdata.Metrics{
 			Name:        pm.GetName(),
 			Description: pm.GetHelp(),
@@ -99,10 +103,14 @@
 			newMetric.Data = convertGauge(pm.GetMetric(), now)
 		case dto.MetricType_COUNTER:
 			newMetric.Data = convertCounter(pm.GetMetric(), now)
-		case dto.MetricType_HISTOGRAM:
-			newMetric.Data = convertHistogram(pm.GetMetric(), now)
 		case dto.MetricType_SUMMARY:
 			newMetric.Data = convertSummary(pm.GetMetric(), now)
+		case dto.MetricType_HISTOGRAM:
+			if isExponentialHistogram(pm.GetMetric()[0].GetHistogram()) {
+				newMetric.Data = convertExponentialHistogram(pm.GetMetric(), now)
+			} else {
+				newMetric.Data = convertHistogram(pm.GetMetric(), now)
+			}
 		default:
 			// MetricType_GAUGE_HISTOGRAM, MetricType_UNTYPED
 			errs = append(errs, fmt.Errorf("%w: %v for metric %v", errUnsupportedType, pm.GetType(), pm.GetName()))
@@ -113,6 +121,16 @@
 	return otelMetrics, errs.errOrNil()
 }
 
+func isExponentialHistogram(hist *dto.Histogram) bool {
+	// The prometheus go client ensures at least one of these is non-zero
+	// so it can be distinguished from a fixed-bucket histogram.
+	// https://github.com/prometheus/client_golang/blob/7ac90362b02729a65109b33d172bafb65d7dab50/prometheus/histogram.go#L818
+	return hist.GetZeroThreshold() > 0 ||
+		hist.GetZeroCount() > 0 ||
+		len(hist.GetPositiveSpan()) > 0 ||
+		len(hist.GetNegativeSpan()) > 0
+}
+
 func convertGauge(metrics []*dto.Metric, now time.Time) metricdata.Gauge[float64] {
 	otelGauge := metricdata.Gauge[float64]{
 		DataPoints: make([]metricdata.DataPoint[float64], len(metrics)),
@@ -157,8 +175,88 @@
 	return otelCounter
 }
 
+func convertExponentialHistogram(metrics []*dto.Metric, now time.Time) metricdata.ExponentialHistogram[float64] {
+	otelExpHistogram := metricdata.ExponentialHistogram[float64]{
+		DataPoints:  make([]metricdata.ExponentialHistogramDataPoint[float64], len(metrics)),
+		Temporality: metricdata.CumulativeTemporality,
+	}
+	for i, m := range metrics {
+		dp := metricdata.ExponentialHistogramDataPoint[float64]{
+			Attributes:    convertLabels(m.GetLabel()),
+			StartTime:     processStartTime,
+			Time:          now,
+			Count:         m.GetHistogram().GetSampleCount(),
+			Sum:           m.GetHistogram().GetSampleSum(),
+			Scale:         m.GetHistogram().GetSchema(),
+			ZeroCount:     m.GetHistogram().GetZeroCount(),
+			ZeroThreshold: m.GetHistogram().GetZeroThreshold(),
+			PositiveBucket: convertExponentialBuckets(
+				m.GetHistogram().GetPositiveSpan(),
+				m.GetHistogram().GetPositiveDelta(),
+			),
+			NegativeBucket: convertExponentialBuckets(
+				m.GetHistogram().GetNegativeSpan(),
+				m.GetHistogram().GetNegativeDelta(),
+			),
+			// TODO: Support exemplars
+		}
+		createdTs := m.GetHistogram().GetCreatedTimestamp()
+		if createdTs.IsValid() {
+			dp.StartTime = createdTs.AsTime()
+		}
+		if t := m.GetTimestampMs(); t != 0 {
+			dp.Time = time.UnixMilli(t)
+		}
+		otelExpHistogram.DataPoints[i] = dp
+	}
+	return otelExpHistogram
+}
+
+func convertExponentialBuckets(bucketSpans []*dto.BucketSpan, deltas []int64) metricdata.ExponentialBucket {
+	if len(bucketSpans) == 0 {
+		return metricdata.ExponentialBucket{}
+	}
+	// 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.
+	initialOffset := bucketSpans[0].GetOffset() - 1
+	// We will have one bucket count for each delta, and zeros for the offsets
+	// after the initial offset.
+	lenCounts := int32(len(deltas))
+	for i, bs := range bucketSpans {
+		if i != 0 {
+			lenCounts += bs.GetOffset()
+		}
+	}
+	counts := make([]uint64, lenCounts)
+	deltaIndex := 0
+	countIndex := int32(0)
+	count := int64(0)
+	for i, bs := range bucketSpans {
+		// Do not insert zeroes if this is the first bucketSpan, since those
+		// zeroes are accounted for in the Offset field.
+		if i != 0 {
+			// Increase the count index by the Offset to insert Offset zeroes
+			countIndex += bs.GetOffset()
+		}
+		for j := uint32(0); j < bs.GetLength(); j++ {
+			// Convert deltas to the cumulative number of observations
+			count += deltas[deltaIndex]
+			deltaIndex++
+			// count should always be positive after accounting for deltas
+			if count > 0 {
+				counts[countIndex] = uint64(count)
+			}
+			countIndex++
+		}
+	}
+	return metricdata.ExponentialBucket{
+		Offset: initialOffset,
+		Counts: counts,
+	}
+}
+
 func convertHistogram(metrics []*dto.Metric, now time.Time) metricdata.Histogram[float64] {
-	// TODO: support converting Prometheus "native" histograms to OTel exponential histograms.
 	otelHistogram := metricdata.Histogram[float64]{
 		DataPoints:  make([]metricdata.HistogramDataPoint[float64], len(metrics)),
 		Temporality: metricdata.CumulativeTemporality,
@@ -188,6 +286,10 @@
 }
 
 func convertBuckets(buckets []*dto.Bucket) ([]float64, []uint64, []metricdata.Exemplar[float64]) {
+	if len(buckets) == 0 {
+		// This should never happen
+		return nil, nil, nil
+	}
 	bounds := make([]float64, len(buckets)-1)
 	bucketCounts := make([]uint64, len(buckets))
 	exemplars := make([]metricdata.Exemplar[float64], 0)
@@ -224,8 +326,8 @@
 			dp.StartTime = createdTs.AsTime()
 		}
 		if t := m.GetTimestampMs(); t != 0 {
 			dp.Time = time.UnixMilli(t)
 		}
 		otelSummary.DataPoints[i] = dp
 	}
 	return otelSummary

bridges/prometheus/producer_test.go

@@ -220,6 +220,109 @@ func TestProduce(t *testing.T) {
 				},
 			}},
 		},
+		{
+			name: "exponential histogram",
+			testFn: func(reg *prometheus.Registry) {
+				metric := prometheus.NewHistogram(prometheus.HistogramOpts{
+					Name: "test_exponential_histogram_metric",
+					Help: "An exponential histogram metric for testing",
+					// This enables collection of native histograms in the prometheus client.
+					NativeHistogramBucketFactor: 1.5,
+					ConstLabels: prometheus.Labels(map[string]string{
+						"foo": "bar",
+					}),
+				})
+				reg.MustRegister(metric)
+				metric.Observe(78.3)
+				metric.Observe(2.3)
+				metric.Observe(2.3)
+				metric.Observe(.5)
+				metric.Observe(-78.3)
+				metric.Observe(-.15)
+				metric.Observe(0.0)
+			},
+			expected: []metricdata.ScopeMetrics{{
+				Scope: instrumentation.Scope{
+					Name: scopeName,
+				},
+				Metrics: []metricdata.Metrics{
+					{
+						Name:        "test_exponential_histogram_metric",
+						Description: "An exponential histogram metric for testing",
+						Data: metricdata.ExponentialHistogram[float64]{
+							Temporality: metricdata.CumulativeTemporality,
+							DataPoints: []metricdata.ExponentialHistogramDataPoint[float64]{
+								{
+									Count:     7,
+									Sum:       4.949999999999994,
+									Scale:     1,
+									ZeroCount: 1,
+									PositiveBucket: metricdata.ExponentialBucket{
+										Offset: -3,
+										Counts: []uint64{1, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1},
+									},
+									NegativeBucket: metricdata.ExponentialBucket{
+										Offset: -6,
+										Counts: []uint64{1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1},
+									},
+									Attributes:    attribute.NewSet(attribute.String("foo", "bar")),
+									ZeroThreshold: prometheus.DefNativeHistogramZeroThreshold,
+								},
+							},
+						},
+					},
+				},
+			}},
+		},
+		{
+			name: "exponential histogram with only positive observations",
+			testFn: func(reg *prometheus.Registry) {
+				metric := prometheus.NewHistogram(prometheus.HistogramOpts{
+					Name: "test_exponential_histogram_metric",
+					Help: "An exponential histogram metric for testing",
+					// This enables collection of native histograms in the prometheus client.
+					NativeHistogramBucketFactor: 1.5,
+					ConstLabels: prometheus.Labels(map[string]string{
+						"foo": "bar",
+					}),
+				})
+				reg.MustRegister(metric)
+				metric.Observe(78.3)
+				metric.Observe(2.3)
+				metric.Observe(2.3)
+				metric.Observe(.5)
+				metric.Observe(0.0)
+			},
+			expected: []metricdata.ScopeMetrics{{
+				Scope: instrumentation.Scope{
+					Name: scopeName,
+				},
+				Metrics: []metricdata.Metrics{
+					{
+						Name:        "test_exponential_histogram_metric",
+						Description: "An exponential histogram metric for testing",
+						Data: metricdata.ExponentialHistogram[float64]{
+							Temporality: metricdata.CumulativeTemporality,
+							DataPoints: []metricdata.ExponentialHistogramDataPoint[float64]{
+								{
+									Count:     5,
+									Sum:       83.39999999999999,
+									Scale:     1,
+									ZeroCount: 1,
+									PositiveBucket: metricdata.ExponentialBucket{
+										Offset: -3,
+										Counts: []uint64{1, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1},
+									},
+									NegativeBucket: metricdata.ExponentialBucket{},
+									Attributes:     attribute.NewSet(attribute.String("foo", "bar")),
+									ZeroThreshold:  prometheus.DefNativeHistogramZeroThreshold,
+								},
+							},
+						},
+					},
+				},
+			}},
+		},
 		{
 			name: "partial success",
 			testFn: func(reg *prometheus.Registry) {
@@ -352,6 +455,7 @@ func TestProduceForStartTime(t *testing.T) {
 					}),
 				})
 				reg.MustRegister(metric)
+				metric.Observe(78.3)
 			},
 			startTimeFn: func(aggr metricdata.Aggregation) []time.Time {
 				dps := aggr.(metricdata.Summary).DataPoints
@@ -362,6 +466,30 @@ func TestProduceForStartTime(t *testing.T) {
 				return sts
 			},
 		},
+		{
+			name: "exponential histogram",
+			testFn: func(reg *prometheus.Registry) {
+				metric := prometheus.NewHistogram(prometheus.HistogramOpts{
+					Name: "test_exponential_histogram_metric",
+					Help: "An exponential histogram metric for testing",
+					// This enables collection of native histograms in the prometheus client.
+					NativeHistogramBucketFactor: 1.5,
+					ConstLabels: prometheus.Labels(map[string]string{
+						"foo": "bar",
+					}),
+				})
+				reg.MustRegister(metric)
+				metric.Observe(78.3)
+			},
+			startTimeFn: func(aggr metricdata.Aggregation) []time.Time {
+				dps := aggr.(metricdata.ExponentialHistogram[float64]).DataPoints
+				sts := make([]time.Time, len(dps))
+				for i, dp := range dps {
+					sts[i] = dp.StartTime
+				}
+				return sts
+			},
+		},
 	}
 	for _, tt := range testCases {
 		t.Run(tt.name, func(t *testing.T) {