[otlp] Update TraceSerializer to use attributes / dropped from state.

[rajkumar-rangaraj]

Fixes # Follow up #5941 (comment)

Changes

Please provide a brief description of the changes here.

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

Attention: Patch coverage is 90.32258% with 3 lines in your changes missing coverage. Please review.

Project coverage is 86.52%. Comparing base (6250307) to head (2094b97).
Report is 371 commits behind head on main.

Files with missing lines	Patch %	Lines
...entation/Serializer/ProtobufOtlpTraceSerializer.cs	90.32%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5946      +/-   ##
==========================================
+ Coverage   83.38%   86.52%   +3.13%     
==========================================
  Files         297      262      -35     
  Lines       12531    11803     -728     
==========================================
- Hits        10449    10212     -237     
+ Misses       2082     1591     -491     

Flag	Coverage Δ
unittests	?
unittests-Project-Experimental	86.50% <90.32%> (?)
unittests-Project-Stable	86.28% <90.32%> (?)

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

Files with missing lines	Coverage Δ
...entation/Serializer/ProtobufOtlpTraceSerializer.cs	89.47% <90.32%> (ø)

... and 243 files with indirect coverage changes

[CodeBlanch]

LGTM