[otlp] Add Trace Exporter to transmit custom serialized data.

[rajkumar-rangaraj]

Fixes Part of #5730,
Design discussion issue #

Changes

Please provide a brief description of the changes here.

This PR introduces the ProtobufOtlpTraceExporter, which will replace the current OtlpTraceExporter in follow-up PRs. I validated that these changes work as expected by adding the environment variable OTEL_DOTNET_EXPERIMENTAL_USE_CUSTOM_PROTOBUF_SERIALIZER to the following example applications, enabling them to send data to Aspire Dashboard:

• Console example
• AspNetCore example

The changes were validated, and everything functions as expected. Please note, I marked ProtobufOtlpTraceExporter as internal.

IProtobufExportClient will replace IExportClient.

Test Changes:

No changes to the tests have been made in this PR. This decision was made to avoid redundant work, as the renaming of ProtobufOtlpTraceExporter to OtlpTraceExporter will happen shortly (within a week of this PR being merged). All tests are expected to work with the new implementation, and I’ve verified this locally by manually adjusting the tests, which confirms that everything functions as expected.

Next Steps:

• Introduce similar changes for Logs and Metrics exporters.
• Replace the existing exporters and update tests as necessary.

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 5.05837% with 244 lines in your changes missing coverage. Please review.

Project coverage is 85.15%. Comparing base (6250307) to head (ecb7ef9).
Report is 391 commits behind head on main.

Files with missing lines	Patch %	Lines
...tlpExporterPersistentStorageTransmissionHandler.cs	0.00%	59 Missing ⚠️
...ation/ExportClient/ProtobufOtlpHttpExportClient.cs	0.00%	45 Missing ⚠️
...ation/ExportClient/ProtobufOtlpGrpcExportClient.cs	0.00%	39 Missing ⚠️
...OpenTelemetryProtocol/ProtobufOtlpTraceExporter.cs	0.00%	39 Missing ⚠️
...mission/ProtobufOtlpExporterTransmissionHandler.cs	0.00%	34 Missing ⚠️
...TelemetryProtocol/OtlpExporterOptionsExtensions.cs	0.00%	16 Missing ⚠️
...on/ProtobufOtlpExporterRetryTransmissionHandler.cs	0.00%	10 Missing ⚠️
...etryProtocol/Implementation/ExperimentalOptions.cs	66.66%	1 Missing ⚠️
...metryProtocol/OtlpTraceExporterHelperExtensions.cs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5969      +/-   ##
==========================================
+ Coverage   83.38%   85.15%   +1.77%     
==========================================
  Files         297      269      -28     
  Lines       12531    12269     -262     
==========================================
- Hits        10449    10448       -1     
+ Misses       2082     1821     -261     

Flag	Coverage Δ
unittests	?
unittests-Project-Experimental	85.09% <5.05%> (?)
unittests-Project-Stable	84.99% <5.05%> (?)

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	89.64% <100.00%> (ø)
...tlpExporterPersistentStorageTransmissionHandler.cs	82.60% <100.00%> (ø)
...ansmission/OtlpExporterRetryTransmissionHandler.cs	80.00% <100.00%> (ø)
...rotocol/Implementation/Transmission/RetryHelper.cs	100.00% <ø> (ø)
...etryProtocol/Implementation/ExperimentalOptions.cs	86.36% <66.66%> (-13.64%)	⬇️
...metryProtocol/OtlpTraceExporterHelperExtensions.cs	96.00% <66.66%> (-0.67%)	⬇️
...on/ProtobufOtlpExporterRetryTransmissionHandler.cs	0.00% <0.00%> (ø)
...TelemetryProtocol/OtlpExporterOptionsExtensions.cs	81.54% <0.00%> (-12.07%)	⬇️
...mission/ProtobufOtlpExporterTransmissionHandler.cs	0.00% <0.00%> (ø)
...ation/ExportClient/ProtobufOtlpGrpcExportClient.cs	0.00% <0.00%> (ø)
... and 3 more

... and 240 files with indirect coverage changes

[CodeBlanch]

LGTM

[CodeBlanch]

@rajkumar-rangaraj Before you remove all the existing stuff make sure to capture some benchmarks so we can show the new version is better (faster, less/non-allocating) 😄