otlpmetric: exponential histogram export drops ZeroThreshold

[pellared]

Description

The OTLP metric transformation for exponential histograms copies Scale and ZeroCount but does not copy metricdata.ExponentialHistogramDataPoint.ZeroThreshold into the OTLP ExponentialHistogramDataPoint. Consequently, both the gRPC and HTTP metric exporters always transmit the protobuf default of 0, even when a producer supplies a non-zero threshold.

This loses part of the metric data and changes how the receiving system interprets the zero bucket. The existing transform fixtures already use input thresholds of .01 and .02, but the expected protobuf fixtures omit the field. Exponential histogram transformation was introduced in #4222.

Environment

• OS: Linux (the behavior is platform-independent)
• Architecture: amd64
• Go Version: 1.26.4
• opentelemetry-go version: 0de413a318cb52629baefb89a1554a905e105aa3

Steps To Reproduce

Add this regression test to internal/shared/otlp/otlpmetric/transform/metricdata_test.go.tmpl and regenerate the gRPC and HTTP transform packages:

func TestExponentialHistogramDataPointsPreservesZeroThreshold(t *testing.T) {
	const want = 0.01
	got := ExponentialHistogramDataPoints(
		[]metricdata.ExponentialHistogramDataPoint[float64]{{ZeroThreshold: want}},
	)
	require.Len(t, got, 1)
	assert.Equal(t, want, got[0].ZeroThreshold)
}

Run the generated tests for both exporters:

cd exporters/otlp/otlpmetric/otlpmetricgrpc
go test ./internal/transform -run '^TestExponentialHistogramDataPointsPreservesZeroThreshold$' -count=1

cd ../otlpmetrichttp
go test ./internal/transform -run '^TestExponentialHistogramDataPointsPreservesZeroThreshold$' -count=1

Both tests currently fail with an expected value of 0.01 and an actual value of 0.

Expected behavior

Both OTLP metric exporters should preserve ZeroThreshold exactly when converting an exponential histogram data point to OTLP.

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