[otlp] Avoid filling new buffer on expansion

[rajkumar-rangaraj]

Fixes #
Design discussion issue #

Changes

Please provide a brief description of the changes here.

This update optimizes the buffer expansion logic by preventing unnecessary re-filling of the newly allocated buffer during retries.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 86.48%. Comparing base (75a683e) to head (8e1195c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6019      +/-   ##
==========================================
- Coverage   86.51%   86.48%   -0.03%     
==========================================
  Files         257      257              
  Lines       11689    11693       +4     
==========================================
  Hits        10113    10113              
- Misses       1576     1580       +4     

Flag	Coverage Δ
unittests-Project-Experimental	86.40% <100.00%> (+0.15%)	⬆️
unittests-Project-Stable	86.33% <100.00%> (+<0.01%)	⬆️

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

Files with missing lines	Coverage Δ
...ementation/Serializer/ProtobufOtlpLogSerializer.cs	98.49% <100.00%> (+0.02%)	⬆️
...ntation/Serializer/ProtobufOtlpMetricSerializer.cs	98.78% <100.00%> (+0.01%)	⬆️
...entation/Serializer/ProtobufOtlpTraceSerializer.cs	95.01% <100.00%> (+0.03%)	⬆️
...ol/Implementation/Serializer/ProtobufSerializer.cs	93.07% <100.00%> (-0.11%)	⬇️

... and 3 files with indirect coverage changes

[CodeBlanch]

LGTM