[BUG] Panic converting OTLP exponential histograms with low scale

[gillesbergerp]

Agent version

opentelemetry-mapping-go v0.29.0

Bug Report

A producer that downscales to its floor takes the whole collector down — the panic lands in an exporter-queue goroutine, so nothing upstream can recover it. scale down to -10 is spec-valid, so this is well-formed input.

gamma in exponentialHistogramToDDSketch is computed but never bounded:

datadog-agent/pkg/opentelemetry-mapping-go/otlp/metrics/default_mapper.go

Line 532 in 0f40baf

gamma := math.Pow(2, math.Pow(2, float64(-p.Scale())))

histograms.go already clamps the identical line so the same guard on the OTLP path should be enough:

datadog-agent/pkg/opentelemetry-mapping-go/otlp/metrics/histograms.go

Lines 178 to 180 in 0f40baf

	gammaWithOnePercentAccuracy := 1.01 / 0.99
	gamma := math.Pow(2, math.Pow(2, float64(-scale)))
	gamma = math.Min(gamma, gammaWithOnePercentAccuracy)

Reproduction Steps

Observed datapoint

-> Name: test.historgram
-> Unit: s
-> DataType: ExponentialHistogram
-> AggregationTemporality: Delta
Count: 3   Sum: 0.627158   Min: 0.143741   Max: 0.327218

161 buckets, reconstructed boundaries:
   85x  (0.000000, 0.000000]
   72x  (+Inf, +Inf]
    1x  [-1.34e152, -1.000000)
    1x  (0.000000, 1.000000]
    1x  (1.000000, 1.34e152]
    1x  (1.34e152, +Inf]

Stack trace

panic: runtime error: slice bounds out of range [-4611686018427387209:]

goroutine 275 [running]:
sketches-go@v1.4.7/ddsketch/store.(*DenseStore).shiftCounts        dense_store.go:115
sketches-go@v1.4.7/ddsketch/store.(*DenseStore).centerCounts(0xc00217d300, 0x8000000000000000, 0x5ea4)
                                                                   dense_store.go:107
sketches-go@v1.4.7/ddsketch/store.(*DenseStore).extendRange(0xc00217d300, 0x7ff0000000000000?, 0x0?)
                                                                   dense_store.go:95
sketches-go@v1.4.7/ddsketch/store.(*DenseStore).AddWithCount(0xc00217d300, 0x8000000000000000, 0xfff8000000000000)
                                                                   dense_store.go:55
sketches-go@v1.4.7/ddsketch.(*DDSketch).ChangeMapping              ddsketch.go:486
opentelemetry-mapping-go/pkg/quantile.createDDSketchWithSketchMapping   ddsketch.go:52
opentelemetry-mapping-go/pkg/quantile.ConvertDDSketchIntoSketch         ddsketch.go:200
opentelemetry-mapping-go/pkg/otlp/metrics.(*Translator).mapExponentialHistogramMetrics
                                                exponential_histograms_translator.go:148
opentelemetry-mapping-go/pkg/otlp/metrics.(*Translator).mapToDDFormat   metrics_translator.go:888
opentelemetry-mapping-go/pkg/otlp/metrics.(*Translator).MapMetrics      metrics_translator.go:827
datadog-agent/.../serializerexporter.(*Exporter).ConsumeMetrics         exporter.go:192
[... exporterhelper frames ...]
created by ...queue.(*asyncQueue[...]).Start in goroutine 1  async_queue.go:39

Agent configuration

• otelcol: v0.129.0
• go.opentelemetry.io/collector/exporter: v0.129.0
• go.opentelemetry.io/collector/consumer: v1.35.0
• DataDog/opentelemetry-mapping-go/pkg/otlp/metrics: v0.29.0
• DataDog/opentelemetry-mapping-go/pkg/quantile: v0.29.0
• DataDog/sketches-go: v1.4.7
• DataDog/datadog-agent/.../serializerexporter: v0.68.0-devel.0.20250604170009-feaecfbfd700

Operating System

No response

Other environment details

No response

[alaodd]

I'm looking into this now