[geneva] Tweak OtlpProtobufSerializer to access MetricPoint by ref

[CodeBlanch]

Changes

• Fix a few spots in GenevaExporter where MetricPoint wasn't being accessed/passed by ref

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 10 lines in your changes missing coverage. Please review.

Project coverage is 0.00%. Comparing base (71655ce) to head (b6bfff5).
Report is 595 commits behind head on main.

Files with missing lines	Patch %	Lines
...eva/Metrics/OtlpProtobuf/OtlpProtobufSerializer.cs	0.00%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #2310       +/-   ##
==========================================
- Coverage   73.91%   0.00%   -73.92%     
==========================================
  Files         267      44      -223     
  Lines        9615    3899     -5716     
==========================================
- Hits         7107       0     -7107     
- Misses       2508    3899     +1391     

Flag	Coverage Δ
unittests-Exporter.Geneva	0.00% <0.00%> (?)

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

Files with missing lines	Coverage Δ
...eva/Metrics/OtlpProtobuf/OtlpProtobufSerializer.cs	0.00% <0.00%> (ø)

... and 264 files with indirect coverage changes

[rajkumar-rangaraj]

LGTM