[CODE HEALTH] Fix clang-tidy bugprone-unused-local-non-trivial-variable warnings

[thc1006]

Resolves the bugprone-unused-local-non-trivial-variable warnings tracked in #4198 (part of #2053). 12 warnings across 6 files.

• exporters/prometheus/src/exporter_utils.cc: removed two dead local copies (origin_name, unit); the loop already uses instrument_descriptor.name_ / .unit_ directly.
• api/test/baggage/baggage_benchmark.cc: the two ToHeader() results are the benchmarked work but were unused; wrapped them in benchmark::DoNotOptimize(...) so the calls are not optimized away and the locals are no longer flagged.
• examples/otlp/{file_metric,http_instrumented,http_metric}_main.cc: removed unused version / schema locals.
• exporters/otlp/test/otlp_file_client_test.cc: removed two unused output_stream locals in the file-rotation tests (the ostream test keeps its own).

Ratchet lowered 328 -> 316 (abiv1-preview) and 338 -> 326 (abiv2-preview).

Verified locally: IWYU (all-options-abiv1-preview) clean, clang-format fixed point; the 12 warnings are the complete set for this check per the main clang-tidy CI report.

Fixes #4198

[Copilot]

Pull request overview

This PR addresses clang-tidy bugprone-unused-local-non-trivial-variable warnings across the SDK/tests/examples, reducing the clang-tidy warning ratchet thresholds accordingly as part of ongoing clang-tidy cleanup.

Changes:

• Remove dead/unused local variables in Prometheus exporter utils, OTLP examples, and OTLP file client tests.
• Ensure Baggage::ToHeader() benchmark work isn’t optimized away by wrapping the call in benchmark::DoNotOptimize(...).
• Update the clang-tidy GitHub Actions warning limits (ratchet) and add a changelog entry.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
exporters/prometheus/src/exporter_utils.cc	Removes unused local copies of instrument name/unit.
api/test/baggage/baggage_benchmark.cc	Prevents benchmarked ToHeader() work from being optimized away.
examples/otlp/file_metric_main.cc	Removes unused version/schema locals.
examples/otlp/http_metric_main.cc	Removes unused version/schema locals.
examples/otlp/http_instrumented_main.cc	Removes unused version/schema locals.
exporters/otlp/test/otlp_file_client_test.cc	Removes unused stringstream locals in rotation tests.
CHANGELOG.md	Adds an entry for this code health change (currently contains a placeholder link).
.github/workflows/clang-tidy.yaml	Lowers warning limits to reflect the removed warnings.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.88%. Comparing base (7ffb5ec) to head (c9bec9d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4202      +/-   ##
==========================================
- Coverage   82.90%   82.88%   -0.01%     
==========================================
  Files         405      405              
  Lines       17289    17287       -2     
==========================================
- Hits        14331    14327       -4     
- Misses       2958     2960       +2     

Files with missing lines	Coverage Δ
exporters/prometheus/src/exporter_utils.cc	89.11% <ø> (-0.06%)	⬇️

... and 1 file with indirect coverage changes

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[marcalff]

LGTM

[thc1006]

Dear reviewer
I have fix the Merge conflict.
now is clean to merge

[marcalff]

LGTM, thanks for the cleanup.