[Rajkaran-122]

Implement OTEL_TRACES_SAMPLER and OTEL_TRACES_SAMPLER_ARG environment variables to configure the trace sampler. This allows users to configure sampling behavior without code changes.

Supported sampler values:

• always_on
• always_off
• traceidratio
• parentbased_traceidratio
• parentbased_always_on
• parentbased_always_off

For ratio-based samplers (traceidratio, parentbased_traceidratio), the ratio is specified via OTEL_TRACES_SAMPLER_ARG (default 1.0 if unset).

Implementation details:

• Added sampler_env.h/cc with GetSamplerFromEnv() function
• Updated TracerProvider and TracerContext constructors to use environment-based sampler when no explicit sampler is provided
• Updated TracerProviderFactory and TracerContextFactory to use environment-based sampler
• Invalid sampler names or ratios fall back to AlwaysOnSampler with a warning
• Explicitly supplied samplers always take precedence over environment variables
• Added comprehensive unit tests covering all supported values, edge cases, and error conditions

Closes #4709

[codecov]

Codecov Report

❌ Patch coverage is 98.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.74%. Comparing base (ac7b87a) to head (ad74150).

Files with missing lines	Patch %	Lines
sdk/src/trace/tracer_provider.cc	88.89%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4729      +/-   ##
==========================================
+ Coverage   86.69%   86.74%   +0.06%     
==========================================
  Files         527      528       +1     
  Lines       20902    20945      +43     
==========================================
+ Hits        18119    18167      +48     
+ Misses       2783     2778       -5     

Files with missing lines	Coverage Δ
...k/include/opentelemetry/sdk/trace/tracer_context.h	100.00% <ø> (ø)
sdk/src/trace/sampler_env.cc	100.00% <100.00%> (ø)
sdk/src/trace/tracer_context.cc	83.34% <100.00%> (+0.99%)	⬆️
sdk/src/trace/tracer_provider_factory.cc	100.00% <100.00%> (ø)
sdk/src/trace/tracer_provider.cc	89.25% <88.89%> (-0.52%)	⬇️

... and 2 files with indirect coverage changes

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[mateenali66]

ran it at ad74150f. unset matches main on all three paths, and the six values reach the constructors and both factories. a few things:

1. parentbased_traceidratio with an out of range arg (1.5, -0.5, nan) returns plain AlwaysOnSampler, so a child of an unsampled remote parent gets sampled. the spec says an invalid arg MUST be ignored as if unset, which is ratio 1.0 with the wrapper kept. 0.5abc already ends up there, so the two invalid paths disagree.
2. names are matched case sensitively, so ALWAYS_OFF samples everything. the spec says enum values SHOULD be case-insensitive, and meter_context.cc already does it for the exemplar filter. CaseInsensitiveSamplerNames passes either way.
3. no test sets the variable and then builds a provider or context.
4. CHANGELOG.md:921 has a leftover >>>>>>> upstream/main, and the Windows jobs need env_variables.h in the test for setenv.