prometheus: validate exponential histogram scale range (open-telemetry#6779)

Author: highlyavailable

CHANGELOG.md

@@ -18,6 +18,10 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - Add metric's schema URL as `otel_scope_schema_url` label in `go.opentelemetry.io/otel/exporters/prometheus`. (#5947)
 - Add metric's scope attributes as `otel_scope_[attribute]` labels in `go.opentelemetry.io/otel/exporters/prometheus`. (#5947)
 
+### Fixed
+
+- Validate exponential histogram scale range for Prometheus compatibility in `go.opentelemetry.io/otel/exporters/prometheus`. (#6779)
+
 <!-- Released section -->
 <!-- Don't change this section unless doing release -->
 

exporters/prometheus/exporter.go

@@ -282,6 +282,19 @@ func addExponentialHistogramMetric[N int64 | float64](
 
 		desc := prometheus.NewDesc(name, m.Description, keys, nil)
 
+		// Prometheus native histograms support scales in the range [-4, 8]
+		scale := dp.Scale
+		if scale < -4 {
+			// Reject scales below -4 as they cannot be represented in Prometheus
+			otel.Handle(fmt.Errorf(
+				"exponential histogram scale %d is below minimum supported scale -4, skipping data point",
+				scale))
+			continue
+		}
+		if scale > 8 {
+			scale = 8
+		}
+
 		// 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 {
@@ -308,7 +321,7 @@ func addExponentialHistogramMetric[N int64 | float64](
 			positiveBuckets,
 			negativeBuckets,
 			dp.ZeroCount,
-			dp.Scale,
+			scale,
 			dp.ZeroThreshold,
 			dp.StartTime,
 			values...)

exporters/prometheus/exporter_test.go

@@ -10,6 +10,7 @@ import (
 	"os"
 	"sync"
 	"testing"
+	"time"
 
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/testutil"
@@ -22,6 +23,7 @@ import (
 	"go.opentelemetry.io/otel/attribute"
 	otelmetric "go.opentelemetry.io/otel/metric"
 	"go.opentelemetry.io/otel/sdk/metric"
+	"go.opentelemetry.io/otel/sdk/metric/metricdata"
 	"go.opentelemetry.io/otel/sdk/resource"
 	semconv "go.opentelemetry.io/otel/semconv/v1.26.0"
 	"go.opentelemetry.io/otel/trace"
@@ -1180,3 +1182,89 @@ func TestExemplars(t *testing.T) {
 		})
 	}
 }
+
+func TestExponentialHistogramScaleValidation(t *testing.T) {
+	ctx := context.Background()
+
+	t.Run("normal_exponential_histogram_works", func(t *testing.T) {
+		registry := prometheus.NewRegistry()
+		exporter, err := New(WithRegisterer(registry), WithoutTargetInfo(), WithoutScopeInfo())
+		require.NoError(t, err)
+
+		provider := metric.NewMeterProvider(
+			metric.WithReader(exporter),
+			metric.WithResource(resource.Default()),
+		)
+		defer func() {
+			err := provider.Shutdown(ctx)
+			require.NoError(t, err)
+		}()
+
+		// Create a histogram with a valid scale
+		meter := provider.Meter("test")
+		hist, err := meter.Float64Histogram(
+			"test_exponential_histogram",
+			otelmetric.WithDescription("test histogram"),
+		)
+		require.NoError(t, err)
+		hist.Record(ctx, 1.0)
+		hist.Record(ctx, 10.0)
+		hist.Record(ctx, 100.0)
+
+		metricFamilies, err := registry.Gather()
+		require.NoError(t, err)
+		assert.NotEmpty(t, metricFamilies)
+	})
+
+	t.Run("error_handling_for_invalid_scales", func(t *testing.T) {
+		var capturedError error
+		originalHandler := otel.GetErrorHandler()
+		otel.SetErrorHandler(otel.ErrorHandlerFunc(func(err error) {
+			capturedError = err
+		}))
+		defer otel.SetErrorHandler(originalHandler)
+
+		now := time.Now()
+		invalidScaleData := metricdata.ExponentialHistogramDataPoint[float64]{
+			Attributes:    attribute.NewSet(),
+			StartTime:     now,
+			Time:          now,
+			Count:         1,
+			Sum:           10.0,
+			Scale:         -5, // Invalid scale below -4
+			ZeroCount:     0,
+			ZeroThreshold: 0.0,
+			PositiveBucket: metricdata.ExponentialBucket{
+				Offset: 1,
+				Counts: []uint64{1},
+			},
+			NegativeBucket: metricdata.ExponentialBucket{
+				Offset: 1,
+				Counts: []uint64{},
+			},
+		}
+
+		ch := make(chan prometheus.Metric, 10)
+		defer close(ch)
+
+		histogram := metricdata.ExponentialHistogram[float64]{
+			Temporality: metricdata.CumulativeTemporality,
+			DataPoints:  []metricdata.ExponentialHistogramDataPoint[float64]{invalidScaleData},
+		}
+
+		m := metricdata.Metrics{
+			Name:        "test_histogram",
+			Description: "test",
+		}
+
+		addExponentialHistogramMetric(ch, histogram, m, "test_histogram", keyVals{})
+		assert.Error(t, capturedError)
+		assert.Contains(t, capturedError.Error(), "scale -5 is below minimum")
+		select {
+		case <-ch:
+			t.Error("Expected no metrics to be produced for invalid scale")
+		default:
+			// No metrics were produced for the invalid scale
+		}
+	})
+}