[otlp] Refactor shared protobuf otlp export client code into a base class

[CodeBlanch]

Changes

• Introduce a base class for ProtobufOtlpGrpcExportClient & ProtobufOtlpHttpExportClient to share to reduce code duplication
• Add a comment explaining why ProtobufOtlpGrpcExportClient doesn't try to do synchronous send

Merge requirement checklist

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

[codecov]

Codecov Report

Attention: Patch coverage is 86.66667% with 6 lines in your changes missing coverage. Please review.

Project coverage is 85.07%. Comparing base (88d2ad6) to head (d5c4aee).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ation/ExportClient/ProtobufOtlpGrpcExportClient.cs	44.44%	5 Missing ⚠️
...mentation/ExportClient/ProtobufOtlpExportClient.cs	97.05%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6001      +/-   ##
==========================================
+ Coverage   85.02%   85.07%   +0.05%     
==========================================
  Files         275      276       +1     
  Lines       12710    12693      -17     
==========================================
- Hits        10807    10799       -8     
+ Misses       1903     1894       -9     

Flag	Coverage Δ
unittests-Project-Experimental	85.03% <86.66%> (+0.02%)	⬆️
unittests-Project-Stable	84.90% <86.66%> (-0.10%)	⬇️

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

Files with missing lines	Coverage Δ
...ation/ExportClient/ProtobufOtlpHttpExportClient.cs	82.35% <100.00%> (-8.56%)	⬇️
...mentation/ExportClient/ProtobufOtlpExportClient.cs	97.05% <97.05%> (ø)
...ation/ExportClient/ProtobufOtlpGrpcExportClient.cs	37.93% <44.44%> (-11.62%)	⬇️

... and 3 files with indirect coverage changes