[otlp] Add experimental feature flag for enabling retries

[vishweshbankwar]

Towards #1779
Design discussion issue #

Changes

Adds OTEL_DOTNET_EXPERIMENTAL_OTLP_ENABLE_INMEMORY_RETRY to enable retries in case of transient errors.

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 84.87%. Comparing base (6250307) to head (0b4496c).
Report is 129 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5435      +/-   ##
==========================================
+ Coverage   83.38%   84.87%   +1.48%     
==========================================
  Files         297      284      -13     
  Lines       12531    12224     -307     
==========================================
- Hits        10449    10375      -74     
+ Misses       2082     1849     -233     

Flag	Coverage Δ
unittests	?
unittests-Solution-Experimental	84.85% <100.00%> (?)
unittests-Solution-Stable	84.62% <100.00%> (?)

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

Files	Coverage Δ
...etryProtocol/Implementation/ExperimentalOptions.cs	100.00% <100.00%> (ø)
...orter.OpenTelemetryProtocol/OtlpExporterOptions.cs	97.08% <100.00%> (+0.72%)	⬆️
...TelemetryProtocol/OtlpExporterOptionsExtensions.cs	94.82% <100.00%> (+1.21%)	⬆️
....Exporter.OpenTelemetryProtocol/OtlpLogExporter.cs	91.17% <100.00%> (+2.28%)	⬆️
...lemetryProtocol/OtlpLogExporterHelperExtensions.cs	70.88% <ø> (-0.19%)	⬇️
...porter.OpenTelemetryProtocol/OtlpMetricExporter.cs	83.87% <100.00%> (+0.53%)	⬆️
...nTelemetryProtocol/OtlpMetricExporterExtensions.cs	99.05% <100.00%> (+0.04%)	⬆️
...xporter.OpenTelemetryProtocol/OtlpTraceExporter.cs	88.88% <100.00%> (-2.29%)	⬇️
...metryProtocol/OtlpTraceExporterHelperExtensions.cs	96.87% <100.00%> (+0.20%)	⬆️

... and 58 files with indirect coverage changes

[CodeBlanch]

LGTM

[utpilla]

LGTM with a CHANGELOG update.