[vttablet] Emit Constant “userLabelDisabled” When skip-user-metrics is Enabled

[harshit-gangal]

Description

This PR updates Vitess to emit a constant "UserLabelDisabled" instead of per-user labels when skip-user-metrics is enabled. This prevents metric explosion in environments with a high number of unique users, reducing unnecessary cardinality while preserving metric usefulness.

Related Issue(s)

• Closes Prevent username metric explosion #18086

Checklist

• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 90.19608% with 5 lines in your changes missing coverage. Please review.

Project coverage is 67.53%. Comparing base (b0352bd) to head (b8dc249).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vttablet/tabletserver/query_executor.go	90.00%	3 Missing ⚠️
go/vt/vttablet/tabletserver/stateful_connection.go	90.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18085      +/-   ##
==========================================
- Coverage   67.61%   67.53%   -0.09%     
==========================================
  Files        1600     1600              
  Lines      260810   261338     +528     
==========================================
+ Hits       176351   176485     +134     
- Misses      84459    84853     +394     

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