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Exemplar keeps stale span context when a reservoir cell is overwritten outside a span

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
88/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
java
Domain
observability

Research direction

Start in sdk/metrics/.../internal/exemplar/ReservoirCell.java at offerMeasurement(), especially lines 73-76. Reproduce with always-on exemplars by recording inside a sampled span and then outside one in the same bucket, then collect. Done when the overwritten exemplar's span context belongs to the new measurement and is invalid for the outside-span recording.

Written by the indexing model from the issue text.

Description

Describe the bug
When an exemplar reservoir cell is overwritten within one collection cycle by a measurement recorded outside a span, the exported exemplar has the new value, time and attributes but keeps the trace/span id of the previous measurement.

Steps to reproduce

  1. Build an SdkMeterProvider with setExemplarFilter(ExemplarFilter.alwaysOn()) (or otel.metrics.exemplar.filter=always_on).
  2. On a histogram with default buckets, call record(1.0) inside a sampled span, then record(2.0) with no active span (same bucket).
  3. Collect.

What did you expect to see?
An exemplar with value=2.0 and an invalid span context. The spec defines an exemplar as one recorded measurement, with the trace/span id of the span active when it was recorded.

What did you see instead?
value=2.0 with the span id of the first measurement. ReservoirCell.offerMeasurement() (sdk/metrics/.../internal/exemplar/ReservoirCell.java lines 73-76) always overwrites value, time and attributes, but updates spanContext only when the new span context is valid.

What version and what artifacts are you using?
Artifacts: opentelemetry-sdk-metrics
Version: main @ ce32c1205
How did you reference these artifacts? Local build

Environment
Compiler: Temurin 21
OS: N/A

Additional context
Not visible with the default trace_based filter, since every measurement it lets through has a sampled span.

Spec: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#exemplar

Dominant language
Java
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