PRS and ERS don't promote replicas taking backups

[ejortegau]

Description

This PR changes ERS and PRS so that they prefer not promoting hosts that are currently taking backups.

The implementation follows what was suggesteed in the RFC of the issue (link below). Namely, the RPCs used to get information about candidates now include an extra field indicating whether they are running backups or not; and that is
used to order the list of promotion candidates.

Related Issue(s)

#16558

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

N/A

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 93.02326% with 3 lines in your changes missing coverage. Please review.

Project coverage is 67.32%. Comparing base (469bdcc) to head (97c0271).
Report is 31 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vttablet/tabletmanager/rpc_backup.go	0.00%	2 Missing ⚠️
go/vt/vtctl/reparentutil/util.go	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16997      +/-   ##
==========================================
+ Coverage   67.31%   67.32%   +0.01%     
==========================================
  Files        1569     1570       +1     
  Lines      252502   252762     +260     
==========================================
+ Hits       169964   170182     +218     
- Misses      82538    82580      +42     

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[frouioui]

It would be good to include an end-to-end test for this covering several cases: only 1 replica in the cluster, and more than one replica, all replicas being backed up, etc.

Moreover, even though this is not a breaking change per se, we should still document it in the v22.0 release notes. Which should be put in ./changelog/22.0/22.0.0/summary.md, the file does not exist yet.

[timvaillancourt]

@ejortegau I think this comment is no longer accurate

[ejortegau]

No, it's still accurate. It reflects the fact that PRS will not promote a tablet that is taking a backup.

[ejortegau]

I have updated the PR to do the following:

1. In the ERS case, filterValidCandidates() does not return hosts taking backups, unless there are no better candidates not taking backups. This means ERS can promote hosts taking backups, but only if there are no other candidates that are not.
2. In the PRS case, ElectNewPrimary() excludes hosts taking backups. This means PRS can fail if there's only one good candidate and it's taking a backup.

Please have another look.

[GuptaManan100]

Just one comment change, rest looks good to me!

[deepthi]

Thanks for the contribution and for your patience with the review process.