sdk/metric: Retain advisory attributes when View has no AttributeFilter

[dashpole]

Fixes #8857.

According to the OpenTelemetry Metrics SDK specification, if attribute_keys is not provided on a View, the SDK should use the 'Attributes' advisory parameter configured on the instrument. Previously, when a View matched an instrument without setting an AttributeFilter, the stream's AttributeFilter remained nil, causing the pipeline to drop the instrument's advisory default attributes.

Centralize AttributeFilter defaulting inside cachedAggregator alongside aggregation and exemplar reservoir provider defaulting.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.4%. Comparing base (1ad6638) to head (4e4c8e5).

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #8859     +/-   ##
=======================================
- Coverage   88.4%   88.4%   -0.1%     
=======================================
  Files        331     331             
  Lines      21001   21001             
=======================================
- Hits       18572   18571      -1     
- Misses      2429    2430      +1     

Files with missing lines	Coverage Δ
sdk/metric/pipeline.go	92.7% <100.0%> (ø)

... and 1 file with indirect coverage changes

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[Copilot]

Pull request overview

This PR fixes a Metrics SDK spec compliance gap in go.opentelemetry.io/otel/sdk/metric: when a View matches an instrument but does not specify attribute_keys (i.e., Stream.AttributeFilter is nil), the SDK now correctly falls back to the instrument’s advisory default attributes (configured via metric/x.WithDefaultAttributes) instead of treating it as “allow all attributes”.

Changes:

• Centralizes Stream.AttributeFilter defaulting in inserter.cachedAggregator, applying advisory default attributes when a View omits an attribute filter.
• Updates pipeline unit tests and adds a meter-level regression test to cover “View matches without AttributeFilter” behavior (and preserves explicit View filter override behavior).
• Adds a changelog entry describing the fix in go.opentelemetry.io/otel/sdk/metric.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
sdk/metric/pipeline.go	Passes instrument advisory keys into cachedAggregator and applies AttributeFilter defaulting there when Views omit it.
sdk/metric/pipeline_test.go	Updates cachedAggregator call sites to match the new signature.
sdk/metric/meter_test.go	Adds regression coverage for matched Views without an attribute filter and validates explicit filter override behavior.
CHANGELOG.md	Documents the user-visible behavior fix in the Unreleased “Fixed” section.

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