Limit exemplar label characters to conform to Prometheus limits

[anuq]

Fixes #6770
Limits exemplar label characters to Prometheus's 128 UTF-8 character maximum. When labels exceed this limit, filtered attributes are dropped while preserving trace_id and span_id for correlation.

Changes

• Added EXEMPLAR_MAX_LABEL_SET_LENGTH constant (128 chars)
• Modified convertExemplar() to check label set length and drop attributes if needed
• Added helper method labelSetLength() to calculate total label character count
• Added comprehensive tests for both within-limit and exceeding-limit scenarios

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.84%. Comparing base (1b207c6) to head (4291d57).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8362      +/-   ##
============================================
+ Coverage     90.82%   90.84%   +0.02%     
- Complexity     7927     7932       +5     
============================================
  Files           895      895              
  Lines         23872    23886      +14     
  Branches       2378     2381       +3     
============================================
+ Hits          21681    21700      +19     
+ Misses         1446     1442       -4     
+ Partials        745      744       -1     

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