Fix race condition that prevents queries from being buffered after vtgate startup

[GuptaManan100]

Description

This PR fixes the race described in #16656.

The fix for the race is to also wait for the keyspace event watcher to have seen the updates for primary tablets being serving if that is something the user has requested to wait on. This fixes the race because before starting query service we can now be certain that the keyspace event watcher has processed the healthcheck responses for primaries of all the shards in the keyspaces. This ensures that we will start buffering if a primary goes non-serving subsequently.

This PR adds unit tests to ensure the behaviour of the newly added functions is as intended. It also adds comments to parts of code it is not necessarily changing but would have facilitated in helping debug the issue faster.

Related Issue(s)

• Fixes Bug Report: Race condition can cause queries to fail when vtgate starts up #16656

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

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 6 lines in your changes missing coverage. Please review.

Project coverage is 68.92%. Comparing base (a54d5ec) to head (d9a0ea0).
Report is 17 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/discovery/keyspace_events.go	90.90%	2 Missing ⚠️
go/vt/srvtopo/discover.go	50.00%	2 Missing ⚠️
go/vt/vtgate/tabletgateway.go	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16655      +/-   ##
==========================================
- Coverage   68.93%   68.92%   -0.02%     
==========================================
  Files        1562     1564       +2     
  Lines      200767   201355     +588     
==========================================
+ Hits       138407   138792     +385     
- Misses      62360    62563     +203     

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[mattlord]

Thank you for working on this, @GuptaManan100 ! I had some questions that hopefully you can help me understand.

[mattlord]

Nice work, @manan! ❤️

[GuptaManan100]

Thank you!

[arthurschreiber]

@GuptaManan100 Should this be backported to earlier releases? 🤔

[GuptaManan100]

It's not really a critical bug fix, so doesn't strictly fall into the category of things we backport. But I don't have strong opinions about it.