[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 95.23810% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.74%. Comparing base (ac7b87a) to head (dd1b751).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
sdk/src/trace/sampler_env.cc	95.92%	2 Missing ⚠️
sdk/src/trace/tracer_provider.cc	88.89%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4729      +/-   ##
==========================================
+ Coverage   86.69%   86.74%   +0.05%     
==========================================
  Files         527      528       +1     
  Lines       20902    20958      +56     
==========================================
+ Hits        18119    18178      +59     
+ Misses       2783     2780       -3     

Files with missing lines	Coverage Δ
...k/include/opentelemetry/sdk/trace/tracer_context.h	100.00% <ø> (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%)	⬇️
sdk/src/trace/sampler_env.cc	95.92% <95.92%> (ø)

... 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.

[mateenali66]

re-ran at dd1b751. points 1, 2 and 4 are fixed. PARENTBASED_TRACEIDRATIO with 1.5 now gives ParentBased{TraceIdRatioBasedSampler{1.000000}} from the provider constructor, the factory and the context factory.

still open:

1. CI is red on this PR's own changes. sampler_env.cc has no newline at the end, which fails the 8 clang builds under -Wnewline-eof. format wants sampler_env.h and tracer_provider.cc, markdown-lint wants * bullets and 80 columns in the CHANGELOG entry, iwyu wants always_on_factory.h out of tracer_context_factory.cc, and clang-tidy wants SamplerEnvTest in an anonymous namespace.
2. still no test that sets the variable and then builds a TracerProvider or TracerContext.
3. the last CHANGELOG bullet and the three ParentBasedTraceIdRatioWith...ReturnsAlwaysOnSampler test names still describe the old fallback.