[METRICS SDK] Enforce a runtime minimum scale for Base2ExponentialHistogramAggregation

[ThomsonTan]

Fixes #4325

Changes

This change enforces a runtime minimum scale of -10 for base2 exponential histograms, guarantees sufficient bucket capacity at that floor to preserve recorded counts, centralizes scale reduction across record, merge, and diff paths, guards the indexer against oversized shifts, and adds regression tests plus changelog documentation for the new behavior.

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

[codecov]

Codecov Report

❌ Patch coverage is 93.02326% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.88%. Comparing base (bbf970a) to head (cf355bd).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...egation/base2_exponential_histogram_aggregation.cc	92.86%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4353      +/-   ##
==========================================
+ Coverage   80.86%   80.88%   +0.02%     
==========================================
  Files         450      450              
  Lines       19215    19247      +32     
==========================================
+ Hits        15537    15566      +29     
- Misses       3678     3681       +3     

Files with missing lines	Coverage Δ
...metry/sdk/metrics/aggregation/aggregation_config.h	100.00% <ø> (ø)
...aggregation/base2_exponential_histogram_indexer.cc	100.00% <100.00%> (ø)
...egation/base2_exponential_histogram_aggregation.cc	95.90% <92.86%> (-0.72%)	⬇️

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