Merge colliding Prometheus label values

[ADITYA-CODE-SOURCE]

Summary

• merge colliding normalized attribute labels by joining values with ';' in original-key order
• preserve existing precedence between metric, resource, and synthetic labels while fixing collision handling
• add regression tests for both metric attribute and allowed resource attribute normalization conflicts

Fixes #8236

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.83%. Comparing base (1b207c6) to head (8f2b4ab).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8364      +/-   ##
============================================
+ Coverage     90.82%   90.83%   +0.01%     
- Complexity     7927     7933       +6     
============================================
  Files           895      895              
  Lines         23872    23899      +27     
  Branches       2378     2378              
============================================
+ Hits          21681    21708      +27     
  Misses         1446     1446              
  Partials        745      745              

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