[SDK] 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 87.80%. Comparing base (031307b) to head (24ab035).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3236      +/-   ##
==========================================
+ Coverage   87.78%   87.80%   +0.02%     
==========================================
  Files         198      198              
  Lines        6308     6324      +16     
==========================================
+ Hits         5537     5552      +15     
- Misses        771      772       +1     

Files with missing lines	Coverage Δ
sdk/src/metrics/state/temporal_metric_storage.cc	97.37% <100.00%> (-0.96%)	⬇️

[marcalff]

Nit: use the aggregation_temporarily local variable, from the previous line, in the if test.

[lalitb]

Thanks done.