feat: Stabilize more views - allow changing aggregation

[cijothomas]

Was initially kept under feature flag to keep scope under control. Stabilizing more and more things now!
Also added additional tests as users can now change Aggregations -

1. Added ExponentialHistogram to coverage
2. Added test coverage to validate that we restrict certain aggregations (Counters can be aggregated using LastValue, and Gauges cannot be aggregated as Sum etc.) - There is no real spec behind this, but this is to avoid producing semantically correct aggregation. We could remove this restriction, if there are actual use cases behind them. Don't think it'd be a breaking change.

[cijothomas]

@ArniDagur Could you review, since you recently worked in this area!

[codecov]

Codecov Report

❌ Patch coverage is 96.79666% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.8%. Comparing base (092983f) to head (5b38106).

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/mod.rs	96.3%	23 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3322     +/-   ##
=======================================
+ Coverage   81.1%   81.8%   +0.7%     
=======================================
  Files        129     129             
  Lines      23719   24366    +647     
=======================================
+ Hits       19238   19938    +700     
+ Misses      4481    4428     -53     

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[Copilot]

Pull request overview

This PR stabilizes more of the metrics Views surface area by making aggregation customization available without the spec_unstable_metrics_views feature flag, and aligns view behavior with the spec by falling back to default aggregation when matching views are incompatible.

Changes:

• Make Aggregation and StreamBuilder::with_aggregation() usable without spec_unstable_metrics_views.
• Update view resolution to “proceed as if the View did not match” when all matching views produce incompatible aggregators (with a warning).
• Add tests covering aggregation overrides via views (including exponential histogram) and incompatible aggregation fallback behavior.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
opentelemetry-sdk/src/metrics/pipeline.rs	Adjusts view matching behavior to ignore incompatible views and fall back to default aggregation; adds a warning log.
opentelemetry-sdk/src/metrics/mod.rs	Removes feature-gating around aggregation exports and adds multiple tests for view aggregation behavior.
opentelemetry-sdk/src/metrics/internal/histogram.rs	Removes feature-gated boundary sanitization logic now that boundaries are validated earlier.
opentelemetry-sdk/src/metrics/instrument.rs	Removes feature-gating for with_aggregation() and a related test gate.
opentelemetry-sdk/CHANGELOG.md	Documents stabilization of Aggregation and StreamBuilder::with_aggregation().
examples/metrics-advanced/Cargo.toml	Updates the example to use stable metrics feature instead of the unstable views feature.

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[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

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[Copilot]

The new early-return condition if matched && !measures.is_empty() breaks Aggregation::Drop views. When a view matches and selects Aggregation::Drop, cached_aggregator returns Ok(None) and the code continues, leaving measures empty and errs empty. This then falls through to the implicit default view and creates a default aggregator, so the instrument is no longer dropped.

Consider treating Ok(None) as a successful view application: only fall back to the default when matched && measures.is_empty() && !errs.is_empty() (i.e., all matching views failed), and otherwise return Ok(measures) even if empty.

[Copilot]

PR description says we "restrict certain aggregations (Counters can be aggregated using LastValue...)", but this test (and is_aggregator_compatible) treats LastValue as incompatible for Counter and falls back to Sum. Please update the PR description to match the implemented behavior (or adjust the behavior if the description is the intent).