vtgate: support filtering tablets by tablet-tags

[timvaillancourt]

Description

This PR implements the RFC: #15908 by:

1. Adding the vtgate flag: --tablet-filter-tags StringMap for a list of tablet-tag KVs to match
2. Considering this optional filter in the healthcheck code

This support works in addition (a secondary filter) to the mutually-exclusive --tablet_filter / --keyspaces_to_watch features

If/when there is agreement on the approach I will add documentation for the new support and flag

Related Issue(s)

Resolves #15908

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 4 lines in your changes are missing coverage. Please review.

Project coverage is 68.24%. Comparing base (f118ba2) to head (2f7a0a1).
Report is 134 commits behind head on main.

Files	Patch %	Lines
go/vt/discovery/healthcheck.go	42.85%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15911      +/-   ##
==========================================
- Coverage   68.40%   68.24%   -0.17%     
==========================================
  Files        1556     1541      -15     
  Lines      195121   197188    +2067     
==========================================
+ Hits       133479   134562    +1083     
- Misses      61642    62626     +984     

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[deepthi]

In general I tend to be thrifty about adding new flags, but this seems like a logical extension of the tags feature of the tablet (which already exists for whatever reason), so I'm not objecting to this.
The implementation and unit tests look fine. I'll leave it to you to investigate and fix the unit tests.

[frouioui]

Left a comment on something i am not sure about. Otherwise, besides the failing tests LGTM.

We should probably do a single website PR that gathers the new flags introduced by this PR and #15919.

[frouioui]

Shouldn't we reset the filters slice here? Otherwise we keep appending the same filters over and over again for each cells.

[timvaillancourt]

@frouioui good catch! yes, this should be a reset each iteration of the loop, updated 👍

[timvaillancourt]

@frouioui re-requested review on the fix 🙇