[release-23.0] repltracker: reset replica lag when we are primary (#18800)

[vitess-bot]

Description

This is a backport of #18800

[vitess-bot]

Review Checklist

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General

• Ensure that the Pull Request has a descriptive title.
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Tests

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Backward compatibility

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.69%. Comparing base (58344e1) to head (d4e220a).
⚠️ Report is 2 commits behind head on release-23.0.

Additional details and impacted files

@@               Coverage Diff                @@
##           release-23.0   #18807      +/-   ##
================================================
- Coverage         69.70%   69.69%   -0.01%     
================================================
  Files              1605     1605              
  Lines            214449   214451       +2     
================================================
- Hits             149475   149472       -3     
- Misses            64974    64979       +5     

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