Use ValueMap in ExpoHistogram

[fraillt]

Fixes #2146

Changes

Refactored ExpoHistogram to use ValueMap for consistency and efficiency.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 95.83333% with 5 lines in your changes missing coverage. Please review.

Project coverage is 79.4%. Comparing base (ebf1bb6) to head (e1e82f9).

Files with missing lines	Patch %	Lines
...-sdk/src/metrics/internal/exponential_histogram.rs	95.8%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2282     +/-   ##
=======================================
- Coverage   79.4%   79.4%   -0.1%     
=======================================
  Files        122     122             
  Lines      20795   20784     -11     
=======================================
- Hits       16514   16503     -11     
  Misses      4281    4281             

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