[sdk-traces] Add support for OTEL_TRACES_SAMPLER and OTEL_TRACES_SAMPLER_ARG

[matt-hensley]

Fixes #3601

Changes

TracerProviders can now have a sampler configured via the OTEL_TRACES_SAMPLER environment variable.

The supported values are: always_off, always_on, traceidratio, parentbased_always_on, parentbased_always_off, and parentbased_traceidratio.

The options traceidratio and parentbased_traceidratio may have the sampler probability configured via the OTEL_TRACES_SAMPLER_ARG environment variable.

For details see: OpenTelemetry Environment Variable Specification.

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 97.61905% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 85.48%. Comparing base (6250307) to head (1ea40e8).
Report is 134 commits behind head on main.

❗ Current head 1ea40e8 differs from pull request most recent head bb35bce. Consider uploading reports for the commit bb35bce to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5448      +/-   ##
==========================================
+ Coverage   83.38%   85.48%   +2.09%     
==========================================
  Files         297      289       -8     
  Lines       12531    12463      -68     
==========================================
+ Hits        10449    10654     +205     
+ Misses       2082     1809     -273     

Flag	Coverage Δ
unittests	?
unittests-Solution-Experimental	85.43% <97.61%> (?)
unittests-Solution-Stable	85.45% <97.61%> (?)

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

Files	Coverage Δ
...nTelemetry/Internal/OpenTelemetrySdkEventSource.cs	80.73% <100.00%> (-1.18%)	⬇️
src/OpenTelemetry/Trace/TracerProviderSdk.cs	99.36% <97.36%> (+0.08%)	⬆️

... and 61 files with indirect coverage changes

[CodeBlanch]

LGTM