[SDK] fix: fixed size exemplar reservoir works correctly.

[proost]

Changes

Looks like current fixed size exemplar reservoir works wrong.

What i fixed:

1. reset the stored reservoir cells by reference not value copied.
2. reset selector state without destroying the selector.
3. reset the simple reservoir’s measurement counter after collection.
4. exclude empty cells from collection results.
5. "ReservoirCellSelector" isn't thread-safe. But OfferMeasurement can be called concurrently so, we should guard it.

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.68%. Comparing base (60c3d11) to head (a978029).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4429      +/-   ##
==========================================
+ Coverage   82.61%   82.68%   +0.07%     
==========================================
  Files         511      511              
  Lines       20132    20137       +5     
==========================================
+ Hits        16631    16648      +17     
+ Misses       3501     3489      -12     

Files with missing lines	Coverage Δ
...k/metrics/exemplar/fixed_size_exemplar_reservoir.h	91.18% <100.00%> (+1.53%)	⬆️
...cs/exemplar/simple_fixed_size_exemplar_reservoir.h	83.34% <100.00%> (+50.01%)	⬆️

... and 3 files with indirect coverage changes

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[lalitb]

This serializes offers and collection for each reservoir. That is the right correctness fix, and the feature is still ENABLE_METRICS_EXEMPLAR_PREVIEW-gated. It does add contention to the exemplar-enabled record path, so please mention that tradeoff in the PR description and open a follow-up to benchmark or explore a less contended design before exemplars become stable.

[proost]

Because resetting "reservoir_cell_selector_" is called in "CollectAndReset". "CollectAndReset" is never called in the this repo, But i can't find what is contract about thread-safe.

It does add contention to the exemplar-enabled record path

Absolutely right. I bit more digging out atomic way.

[lalitb]

The reset is correct, but the sampling calculation just above is still off by one. measurement_num is zero-based, so the random choice must cover [0, measurement_num]. With a size-one reservoir, % measurement_num makes the second measurement replace the first every time instead of with 50% probability; size 0 also reaches modulo zero. Could we fix that here and add a deterministic test for the selection bounds?

[Copilot]

Pull request overview

This PR fixes correctness and thread-safety issues in the preview metrics exemplar fixed-size reservoirs (including the simple fixed-size and aligned histogram bucket variants), ensuring reservoirs can be reused across collection intervals and behave correctly under concurrent OfferMeasurement/CollectAndReset usage.

Changes:

• Serialize OfferMeasurement and CollectAndReset in FixedSizeExemplarReservoir with a mutex, and reset selector state (without destroying it) after collection.
• Fix collection/reset semantics: reset stored cells by reference (not by value-copy iteration) and omit empty (null) exemplar results from CollectAndReset.
• Add unit tests (CMake + Bazel) covering multi-interval correctness, simple reservoir sampling restart, and concurrency serialization; update CHANGELOG.md.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
sdk/test/metrics/exemplar/fixed_size_exemplar_reservoir_test.cc	Adds tests validating interval reuse, selector reset behavior, and offer/collect serialization.
sdk/test/metrics/exemplar/CMakeLists.txt	Registers the new exemplar reservoir test in the CMake test target list.
sdk/test/metrics/exemplar/BUILD	Adds a Bazel cc_test target for the new fixed-size exemplar reservoir tests.
sdk/include/opentelemetry/sdk/metrics/exemplar/simple_fixed_size_exemplar_reservoir.h	Implements selector reset() to restart sampling each collection interval.
sdk/include/opentelemetry/sdk/metrics/exemplar/fixed_size_exemplar_reservoir.h	Fixes reset semantics, filters empty cells, and adds locking to make offer/collect thread-safe.
CHANGELOG.md	Documents the exemplar reservoir fixes as a metrics SDK change.

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