[release-20.0] Fix race in replicationLagModule of go/vt/throttle (#16078)

[vitess-bot]

Description

This is a backport of #16078

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 88.46154% with 3 lines in your changes missing coverage. Please review.

Project coverage is 68.68%. Comparing base (f3313b1) to head (05e0083).
Report is 2 commits behind head on release-20.0.

Files with missing lines	Patch %	Lines
go/vt/throttler/replication_lag_cache.go	91.30%	2 Missing ⚠️
go/vt/throttler/throttler.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release-20.0   #16900      +/-   ##
================================================
+ Coverage         68.66%   68.68%   +0.01%     
================================================
  Files              1541     1541              
  Lines            197964   197981      +17     
================================================
+ Hits             135925   135974      +49     
+ Misses            62039    62007      -32     

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