Optimize Metric Processing for Single Collector with Delta Temporality

[lalitb]

This introduces a fast path for cases with a single collector/reader using delta temporality. It skips unnecessary intermediate processing and directly generates metrics for callback, reducing memory usage and CPU overhead. This change is supposed to improve performance for common single-collector scenarios while retaining existing functionality for other cases.

Changes

Please provide a brief description of the changes here.

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

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.18%. Comparing base (d19eb32) to head (983162b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3236      +/-   ##
==========================================
+ Coverage   88.16%   88.18%   +0.02%     
==========================================
  Files         198      198              
  Lines        6224     6241      +17     
==========================================
+ Hits         5487     5503      +16     
- Misses        737      738       +1     

Files with missing lines	Coverage Δ
sdk/src/metrics/state/temporal_metric_storage.cc	97.41% <100.00%> (-0.93%)	⬇️