VReplication: Take replication lag into account in VStreamManager healthcheck result processing

[mattlord]

Description

The vtgate VStreamManager — which manages VStream API requests made against a vtgate — watches a healthcheck stream from the tablets that it's streaming data from for each high level vstream. When processing the source tablet's healthcheck responses, however, it was not taking replication lag into account as it should. See the issue for more details.

This PR corrects that by checking the replication lag reported in the healthcheck response against the vtgate's live value tied to the --discovery_low_replication_lag flag (but this is also a dynamic variable).

I think that we should backport this all the way to 17.0 as it's a small and straightforward fix for a somewhat glaring bug.

Related Issue(s)

• Fixes: Bug Report: VStream does not switch tablets when lag exceeds discovery_low_replication_lag #15754

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• 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]

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[codecov]

Codecov Report

Attention: Patch coverage is 95.23810% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 68.40%. Comparing base (178e6e8) to head (ed8deea).
Report is 9 commits behind head on main.

Files	Patch %	Lines
go/vt/vtgate/vstream_manager.go	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15761      +/-   ##
==========================================
+ Coverage   68.38%   68.40%   +0.01%     
==========================================
  Files        1556     1556              
  Lines      195361   195437      +76     
==========================================
+ Hits       133598   133689      +91     
+ Misses      61763    61748      -15     

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