SHOW VITESS_REPLICATION_STATUS: Only use replication tracker when it's enabled

[mattlord]

Description

We added the show vitess_replication_status query in #8900. At the time I chose to use the tablet's replication lag stats variable as we could get a more accurate representation of the lag than using mysqld's seconds_behind_[master|source] value. The stat relied on polling — in which case we can also estimate the lag when replication is not running – or sub-second heartbeats, and it was more accurate for these reasons.

This query was first introduced in v12 and at that time the ReplicationTracker was always enabled in the local examples using the polling method, where the manual testing was done. In v17 we changed the vttablet flags in the local examples in order to switch from the polling based method to the heartbeat based method before subsequently removing the heartbeats as well and after that we instead started using on-demand heartbeats alone — so today the replication tracker is disabled entirely in the local examples.

In any event, the real issue was that we didn't have adequate testing of this feature. This PR addresses the issue by:

1. Only using the tablet's tracker based stat when replication tracking is enabled (heartbeat or polling based)
2. Adds an e2e test 🙂

You can see a manual test case here: #15341 (comment)

I think that this should be backported for the following reasons:

1. The changes are simple and isolated to users of this query
2. The results were incorrect for the query

I also updated the throttler check to use check-self as check?app=vtgate is not useful because:

1. check is only applicable on a PRIMARY tablet and we are checking non-PRIMARY tablets
2. An app of vtgate doesn't make sense as vtgate does not interface with the tablet throttler

Related Issue(s)

• Fixes: Bug Report: 'show vitess_replication_status' always says ReplicationLag is 0 #15341

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 81.81818% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 65.43%. Comparing base (27be916) to head (fc5f2f3).
Report is 17 commits behind head on main.

❗ Current head fc5f2f3 differs from pull request most recent head bbcb44a. Consider uploading reports for the commit bbcb44a to get more accurate results

Files	Patch %	Lines
go/vt/vtgate/executor.go	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15348      +/-   ##
==========================================
- Coverage   67.53%   65.43%   -2.11%     
==========================================
  Files        1561     1561              
  Lines      193387   193531     +144     
==========================================
- Hits       130607   126634    -3973     
- Misses      62780    66897    +4117     

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[GuptaManan100]

Rest LGTM