[SDK] refactor: exemplar filters

[proost]

Related to #4178, #2526

Changes

Breaking Change is accompanied, But Because this is preview version, I think acceptable.

I will send another PR for configuration from envs.

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

[codecov]

Codecov Report

❌ Patch coverage is 84.21053% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.65%. Comparing base (3893576) to head (0e5c5de).

Files with missing lines	Patch %	Lines
...ntelemetry/sdk/metrics/state/sync_metric_storage.h	55.56%	4 Missing ⚠️
...entelemetry/sdk/metrics/exemplar/reservoir_utils.h	75.00%	1 Missing ⚠️
...telemetry/sdk/metrics/state/async_metric_storage.h	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4267      +/-   ##
==========================================
+ Coverage   82.49%   82.65%   +0.16%     
==========================================
  Files         502      503       +1     
  Lines       19899    19904       +5     
==========================================
+ Hits        16414    16449      +35     
+ Misses       3485     3455      -30     

Files with missing lines	Coverage Δ
...ude/opentelemetry/sdk/metrics/data/exemplar_data.h	100.00% <100.00%> (+100.00%)	⬆️
...ntelemetry/sdk/metrics/exemplar/filter_predicate.h	100.00% <100.00%> (ø)
...k/metrics/exemplar/fixed_size_exemplar_reservoir.h	89.66% <100.00%> (ø)
...metry/sdk/metrics/exemplar/no_exemplar_reservoir.h	100.00% <100.00%> (ø)
...ude/opentelemetry/sdk/metrics/exemplar/reservoir.h	100.00% <ø> (ø)
...pentelemetry/sdk/metrics/exemplar/reservoir_cell.h	97.83% <100.00%> (+41.31%)	⬆️
sdk/src/metrics/meter.cc	81.36% <ø> (ø)
...entelemetry/sdk/metrics/exemplar/reservoir_utils.h	95.24% <75.00%> (-4.76%)	⬇️
...telemetry/sdk/metrics/state/async_metric_storage.h	93.19% <75.00%> (+2.28%)	⬆️
...ntelemetry/sdk/metrics/state/sync_metric_storage.h	84.94% <55.56%> (+1.19%)	⬆️

... and 2 files with indirect coverage changes

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[lalitb]

@proost - I think this won't yet close #4178 - We can't declare exemplar stable till we have evaluated their performance on hot-path of metrics.

[dbarker]

Thanks for the PR! Please see some initial feedback below.

[proost]

@dbarker @lalitb
I'm sorry to bothering you. I missunderstand the issue and spec.

So this PR change:

1. removing span context check so that alwaysOn works correctly.
2. little bit refactoring(removing useless "GetSimpleFilteredExemplarReservoir")
3. tiny performance improving that removing timestamp.

[Copilot]

🟡 Not ready to approve

There are correctness and robustness issues in the updated exemplar code paths (notably async exemplar attribute handling and reservoir cell reset semantics) that should be addressed before approval.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

This review doesn't count toward merge requirements. Sign up for the private preview to control whether Copilot approvals count.

Pull request overview

This PR updates the Metrics SDK exemplar “preview” pipeline to align with spec changes by moving exemplar eligibility logic into a predicate, simplifying reservoir selection based on the filter type, and removing timestamps from the ExemplarReservoir::OfferMeasurement() API.

Changes:

• Remove the SystemTimestamp parameter from preview ExemplarReservoir::OfferMeasurement() and update all call sites/tests accordingly.
• Centralize exemplar eligibility via ExemplarFilterEnabled(...) and add unit tests for the filter predicate and span-context behavior.
• Adjust exemplar reservoir construction to respect ExemplarFilterType (notably kAlwaysOff) and update build/test wiring (CMake + Bazel).

File summaries

File	Description
sdk/test/metrics/exemplar/with_trace_sample_filter_test.cc	Removes legacy tests tied to the old ExemplarFilter interface.
sdk/test/metrics/exemplar/always_sample_filter_test.cc	Removes legacy tests tied to the old ExemplarFilter interface.
sdk/test/metrics/exemplar/reservoir_cell_test.cc	Adds coverage for producing exemplars without an active span context.
sdk/test/metrics/exemplar/no_exemplar_reservoir_test.cc	Updates tests for the new OfferMeasurement signature (no timestamp).
sdk/test/metrics/exemplar/aligned_histogram_bucket_exemplar_reservoir_test.cc	Updates tests for the new OfferMeasurement signature (no timestamp).
sdk/test/metrics/exemplar/filter_predicate_test.cc	Adds new unit tests for ExemplarFilterEnabled(...) behavior.
sdk/test/metrics/exemplar/CMakeLists.txt	Registers the new filter predicate test target.
sdk/test/metrics/exemplar/BUILD	Registers the new Bazel test target for filter predicate tests.
sdk/src/metrics/meter.cc	Passes the filter type into reservoir selection.
sdk/include/opentelemetry/sdk/metrics/state/sync_metric_storage.h	Uses ExemplarFilterEnabled and removes timestamp from reservoir offers.
sdk/include/opentelemetry/sdk/metrics/state/async_metric_storage.h	Uses ExemplarFilterEnabled and removes timestamp from reservoir offers.
sdk/include/opentelemetry/sdk/metrics/exemplar/reservoir.h	Updates the reservoir interface to drop timestamps and removes filtered-reservoir factory.
sdk/include/opentelemetry/sdk/metrics/exemplar/reservoir_utils.h	Selects NoExemplarReservoir when filter is kAlwaysOff and uses factory helpers.
sdk/include/opentelemetry/sdk/metrics/exemplar/reservoir_cell.h	Changes how span context is captured (always overwritten).
sdk/include/opentelemetry/sdk/metrics/exemplar/no_exemplar_reservoir.h	Updates overrides for the new OfferMeasurement signature.
sdk/include/opentelemetry/sdk/metrics/exemplar/fixed_size_exemplar_reservoir.h	Updates overrides for the new OfferMeasurement signature.
sdk/include/opentelemetry/sdk/metrics/exemplar/filter_predicate.h	Introduces the shared exemplar filter predicate helper.
ci/do_ci.sh	Increases Bazel valgrind test timeout.
CHANGELOG.md	Documents exemplar filtering changes and the breaking preview API update.

Review details

• Files reviewed: 19/19 changed files
• Comments generated: 5
• Review effort level: Lite

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[dbarker]

Thanks for the fix. I think this is reasonable. Please see optional feedback (can be follow up PRs) and consider the copilot feedback. We do still need benchmarking for the exemplar path to resolve the linked ticket.

[lalitb]

@proost Could you please change Fixes #4178 to Related to #4178 in the PR description? This PR improves the exemplar implementation, but we still need hot-path performance evaluation before considering exemplars stable and closing that issue.

[lalitb]

Thanks.