[Metrics SDK] Fix spatial re-aggregation for asynchronous instruments

[nikhilbhatia08]

Fixes #1724

Changes

Fix spatial re-aggregation for asynchronous instruments. A view which drops attributes is now applied to asynchronous instruments as well, and the observations which collapse onto the same attribute set are re-aggregated (summed up for the additive instruments) instead of the last observation overwriting the previous ones. Note that AsyncMetricStorage's constructor now takes the view's AttributesProcessor, matching SyncMetricStorage.

For significant contributions please make sure you have completed the following items:

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

[codecov]

Codecov Report

❌ Patch coverage is 96.42857% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.53%. Comparing base (ad74ba2) to head (7f6100f).

Files with missing lines	Patch %	Lines
...telemetry/sdk/metrics/state/async_metric_storage.h	96.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4635      +/-   ##
==========================================
+ Coverage   86.53%   86.53%   +0.01%     
==========================================
  Files         525      525              
  Lines       20475    20498      +23     
==========================================
+ Hits        17715    17735      +20     
- Misses       2760     2763       +3     

Files with missing lines	Coverage Δ
sdk/src/metrics/meter.cc	81.36% <100.00%> (ø)
...telemetry/sdk/metrics/state/async_metric_storage.h	93.75% <96.30%> (+0.57%)	⬆️

... and 2 files with indirect coverage changes

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