stdoutmetric: WithoutTimestamps mutates ResourceMetrics passed to Export

[pellared]

Description

When stdoutmetric.WithoutTimestamps is enabled, Export calls redactTimestamps on the caller-provided *metricdata.ResourceMetrics. That function replaces every metric's Data value in place with a timestamp-redacted aggregation.

The metric.Exporter contract allows the caller to reuse ResourceMetrics after Export returns. The mutation therefore strips timestamps from data subsequently used by another consumer or exporter. WithoutTimestamps originally copied the data in #3828; the in-place behavior appeared after the export interface changed to a pointer in #3853.

Environment

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

Steps To Reproduce

Add the following test to the stdoutmetric exporter tests:

package stdoutmetric_test

import (
	"io"
	"testing"
	"time"

	"github.com/stretchr/testify/assert"
	"github.com/stretchr/testify/require"

	"go.opentelemetry.io/otel/exporters/stdout/stdoutmetric"
	"go.opentelemetry.io/otel/sdk/metric/metricdata"
)

func TestWithoutTimestampsDoesNotMutateInput(t *testing.T) {
	start := time.Unix(1, 0).UTC()
	end := time.Unix(2, 0).UTC()
	rm := &metricdata.ResourceMetrics{
		ScopeMetrics: []metricdata.ScopeMetrics{{
			Metrics: []metricdata.Metrics{{
				Data: metricdata.Gauge[int64]{
					DataPoints: []metricdata.DataPoint[int64]{{
						StartTime: start,
						Time:      end,
						Value:     1,
					}},
				},
			}},
		}},
	}
	exp, err := stdoutmetric.New(
		stdoutmetric.WithWriter(io.Discard),
		stdoutmetric.WithoutTimestamps(),
	)
	require.NoError(t, err)

	require.NoError(t, exp.Export(t.Context(), rm))
	got := rm.ScopeMetrics[0].Metrics[0].Data.(metricdata.Gauge[int64]).DataPoints[0]
	assert.Equal(t, start, got.StartTime)
	assert.Equal(t, end, got.Time)
}

Run:

cd exporters/stdout/stdoutmetric
go test . -run '^TestWithoutTimestampsDoesNotMutateInput$' -count=1

On current main, both timestamps in the caller's data are zero after Export returns.

Expected behavior

WithoutTimestamps should redact only the encoded output. Export should leave the caller-provided ResourceMetrics unchanged.

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