[CODE HEALTH] Fix clang-tidy warnings in base2 exponential histogram aggregation

[thc1006]

Fixes #3979
Contributes to #2053

Changes

• In GetScaleReduction, widen the loop condition to int64_t so that
  end_index - start_index + 1 cannot overflow int32_t at
  max_scale_ = 20 with extreme-span histograms, and so
  bugprone-misplaced-widening-cast stops firing.
• In Downscale, wrap the point_data_.scale_ -= by assignment with
  static_cast<int32_t>(point_data_.scale_ - by) so the
  int32_t - uint32_t arithmetic result is explicitly narrowed back to
  int32_t.
• In Merge and Diff, apply the same static_cast<int32_t>(...) wrap to
  the five remaining scale_ -= scale_reduction sites.
• Decrement the clang-tidy.yaml warning_limit by 7 for both
  all-options-abiv1-preview and all-options-abiv2-preview matrix
  entries to ratchet the limit down to the new measured count.

The arithmetic is behaviour-preserving: in the six scale_ -= uint32_t sites,
the existing code already relies on two's-complement wrap-around through the
unsigned intermediate, and static_cast<int32_t> reproduces the same bit
pattern. In GetScaleReduction, the int64_t widening gives the same
numeric result on values that fit in int32_t but also handles the
theoretical edge case at max_scale_ = 20 where the bucket range can span
near INT32_MAX.

Existing Base2ExponentialHistogramAggregation* cases in
sdk/test/metrics/aggregation_test.cc cover all modified paths; in
particular MergeCountInvariant uses max_buckets_ = 5 with five-bucket
delta cycles to deliberately exceed the limit and trigger the scale-reduction
branch in Merge (which calls GetScaleReduction and then the three
scale_ -= scale_reduction assignments). The Diff path is covered by the
scale-mismatched calls in the other Base2 tests.

Fixes the following warnings:

---

opentelemetry-cpp/sdk/src/metrics/aggregation/base2_exponential_histogram_aggregation.cc (7 warnings)

Line	Check	Message
35	bugprone-misplaced-widening-cast	either cast from 'int' to 'size_t' (aka 'unsigned long') is ineffective, or there is loss of precision before the conversion
215	cppcoreguidelines-narrowing-conversions	narrowing conversion from 'uint32_t' (aka 'unsigned int') to signed type 'int32_t' (aka 'int') is implementation-defined
327	cppcoreguidelines-narrowing-conversions	narrowing conversion from 'uint32_t' (aka 'unsigned int') to signed type 'int32_t' (aka 'int') is implementation-defined
328	cppcoreguidelines-narrowing-conversions	narrowing conversion from 'uint32_t' (aka 'unsigned int') to signed type 'int32_t' (aka 'int') is implementation-defined
329	cppcoreguidelines-narrowing-conversions	narrowing conversion from 'uint32_t' (aka 'unsigned int') to signed type 'int32_t' (aka 'int') is implementation-defined
412	cppcoreguidelines-narrowing-conversions	narrowing conversion from 'uint32_t' (aka 'unsigned int') to signed type 'int32_t' (aka 'int') is implementation-defined
413	cppcoreguidelines-narrowing-conversions	narrowing conversion from 'uint32_t' (aka 'unsigned int') to signed type 'int32_t' (aka 'int') is implementation-defined

---

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[Copilot]

Pull request overview

This PR addresses clang-tidy warnings in the SDK’s base2 exponential histogram aggregation implementation and tightens CI’s clang-tidy warning thresholds accordingly.

Changes:

• Adjusts GetScaleReduction arithmetic to avoid int32_t overflow in the bucket-span computation.
• Makes scale decrements explicit where scale_ is adjusted by uint32_t reductions (Downscale/Merge/Diff).
• Lowers clang-tidy GitHub Actions warning_limit values to reflect the reduced warning count and adds an Unreleased changelog entry.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
sdk/src/metrics/aggregation/base2_exponential_histogram_aggregation.cc	Arithmetic/casting changes intended to eliminate clang-tidy warnings in scale-reduction and scale-update paths.
CHANGELOG.md	Adds an Unreleased entry for this code health change.
.github/workflows/clang-tidy.yaml	Reduces clang-tidy warning limits in the CI matrix to “ratchet” the baseline downward.

[Copilot]

Pull request overview

This PR addresses clang-tidy findings in the SDK’s base2 exponential histogram aggregation by making integer arithmetic conversions explicit and preventing edge-case overflow in scale-reduction calculations, and then ratchets the CI warning limits accordingly.

Changes:

• Prevent potential int32_t overflow in GetScaleReduction by widening the span computation/comparison to int64_t.
• Make scale_ -= <uint32_t> updates explicit by casting the RHS to int32_t in Downscale, Merge, and Diff to satisfy narrowing-conversion checks.
• Lower clang-tidy CI warning_limit thresholds to reflect the reduced warning count, and add a changelog entry.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
sdk/src/metrics/aggregation/base2_exponential_histogram_aggregation.cc	Adjusts arithmetic types/casts to eliminate clang-tidy warnings and avoid theoretical overflow in bucket-span calculations.
CHANGELOG.md	Adds an Unreleased entry documenting the code health fix.
.github/workflows/clang-tidy.yaml	Lowers warning limits for two matrix presets to match the new measured warning counts.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.33%. Comparing base (a42027a) to head (a86cf23).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3997      +/-   ##
==========================================
+ Coverage   90.18%   90.33%   +0.16%     
==========================================
  Files         230      230              
  Lines        7299     7299              
==========================================
+ Hits         6582     6593      +11     
+ Misses        717      706      -11     

Files with missing lines	Coverage Δ
...egation/base2_exponential_histogram_aggregation.cc	95.87% <100.00%> (+4.55%)	⬆️

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[Copilot]

Pull request overview

This PR addresses clang-tidy warnings in the SDK’s base2 exponential histogram aggregation implementation by making integer widening/narrowing explicit and overflow-safe, and then ratchets the clang-tidy CI warning limits to reflect the reduced warning count.

Changes:

• Adjust GetScaleReduction’s span comparison to use int64_t arithmetic to avoid int32_t overflow and satisfy bugprone-misplaced-widening-cast.
• Make scale_ updates explicitly subtract in the signed domain (static_cast<int32_t>(...)) across Downscale, Merge, and Diff to resolve narrowing-conversion warnings.
• Add a focused unit test to exercise the Diff() downscaling path; update clang-tidy workflow warning limits and changelog.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
sdk/src/metrics/aggregation/base2_exponential_histogram_aggregation.cc	Fixes clang-tidy warnings by widening span arithmetic and ensuring signed-domain subtraction for scale_.
sdk/test/metrics/aggregation_test.cc	Adds a test that forces Diff() to take the scale-reduction/downscale branch.
CHANGELOG.md	Records the code-health change in the Unreleased section.
.github/workflows/clang-tidy.yaml	Lowers clang-tidy warning_limit values to reflect the new measured warning count.

[dbarker]

Looks reasonable. Thanks for the warning fixes and test case!

[lalitb]

LGTM. Thanks.