[OpenTelementry] Fix observable callbacks

[martincostello]

Changes

Fix observable instrument callbacks running once per reader instead of once per collection cycle.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

❌ Patch coverage is 94.73684% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.58%. Comparing base (1bfb26d) to head (1d2970b).
⚠️ Report is 19 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...nTelemetry/Metrics/Reader/CompositeMetricReader.cs	71.42%	2 Missing ⚠️
...emetry/Metrics/Reader/BaseExportingMetricReader.cs	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7188      +/-   ##
==========================================
+ Coverage   89.52%   89.58%   +0.05%     
==========================================
  Files         272      272              
  Lines       13347    13380      +33     
==========================================
+ Hits        11949    11986      +37     
+ Misses       1398     1394       -4     

Flag	Coverage Δ
unittests-Project-Experimental	89.45% <94.73%> (+0.02%)	⬆️
unittests-Project-Stable	89.37% <94.73%> (-0.08%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/OpenTelemetry/Metrics/MeterProviderSdk.cs	94.58% <100.00%> (+0.85%)	⬆️
src/OpenTelemetry/Metrics/Reader/MetricReader.cs	92.10% <100.00%> (+2.39%)	⬆️
...emetry/Metrics/Reader/BaseExportingMetricReader.cs	92.30% <92.85%> (+0.12%)	⬆️
...nTelemetry/Metrics/Reader/CompositeMetricReader.cs	75.36% <71.42%> (+2.22%)	⬆️

... and 5 files with indirect coverage changes

[Copilot]

Pull request overview

Fixes observable instrument callbacks being invoked once per reader (when using multiple readers via CompositeMetricReader) by ensuring observable instruments are collected once per collection cycle and then shared across underlying readers.

Changes:

• Refactors MetricReader.Collect to support a composite collection mode that skips observable instrument collection for child readers.
• Updates CompositeMetricReader to collect observable instruments once per collection, then collect from each child reader without re-invoking callbacks.
• Updates unit tests and changelog to reflect the corrected callback and export 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
test/OpenTelemetry.Tests/Metrics/MultipleReadersTests.cs	Updates expectations to ensure observable callbacks run once per collection and exported values match the new semantics.
src/OpenTelemetry/Metrics/Reader/MetricReader.cs	Introduces composite-aware collection path (CollectFromComposite/OnCollectFromComposite) and centralizes observable collection behind a helper.
src/OpenTelemetry/Metrics/Reader/CompositeMetricReader.cs	Collects observable instruments once, then triggers child readers via composite-specific collection to avoid duplicate callbacks.
src/OpenTelemetry/Metrics/Reader/BaseExportingMetricReader.cs	Overrides OnCollectFromComposite to preserve push/pull gating when collected via a composite reader.
src/OpenTelemetry/Metrics/MeterProviderSdk.cs	Ensures the active reader (including a newly-created composite reader) has its parent provider set, enabling observable collection from composite.
src/OpenTelemetry/CHANGELOG.md	Adds an unreleased entry describing the observable callback fix.

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