[otlp] Correct initial write position for gRPC in .NET Framework OTLP exporters

[pengweiqhca]

…l.Grpc`.

Fixes #6273
Replaces: #6277

Changes

Please provide a brief description of the changes here.

Verify

1. Start the opentelemetry-collector with the OTLP receiver enabled and TLS configured..
2. Debug the Examples.Console.csproj project with -f net48 and the following arguments: otlp -e "https://localhost:4317" -p "grpc"
3. Set a breakpoint in OtlpExporterTransmissionHandler.TrySubmitRequest, and response.Success should return true.

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 86.61%. Comparing base (8c1e638) to head (a2ff1bb).
Report is 1 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6280   +/-   ##
=======================================
  Coverage   86.61%   86.61%           
=======================================
  Files         258      258           
  Lines       11795    11792    -3     
=======================================
- Hits        10216    10214    -2     
+ Misses       1579     1578    -1     

Flag	Coverage Δ
unittests-Project-Experimental	86.44% <ø> (-0.02%)	⬇️
unittests-Project-Stable	86.47% <ø> (+0.04%)	⬆️

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

Files with missing lines	Coverage Δ
....Exporter.OpenTelemetryProtocol/OtlpLogExporter.cs	89.65% <ø> (-0.35%)	⬇️
...porter.OpenTelemetryProtocol/OtlpMetricExporter.cs	80.76% <ø> (-0.72%)	⬇️
...xporter.OpenTelemetryProtocol/OtlpTraceExporter.cs	89.28% <ø> (-0.37%)	⬇️

... and 1 file with indirect coverage changes

[rajkumar-rangaraj]

Thanks @pengweiqhca for your contributions!