PRS and ERS don't promote replicas taking backups

[ejortegau]

Description

This PR prevents ERS and PRS from 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 filter out the list of valid 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 86.11111% with 5 lines in your changes missing coverage. Please review.

Project coverage is 67.03%. Comparing base (e881b9f) to head (d7b8ddd).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtctl/reparentutil/emergency_reparenter.go	77.77%	2 Missing ⚠️
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   67.04%   67.03%   -0.01%     
==========================================
  Files        1571     1571              
  Lines      251677   251700      +23     
==========================================
+ Hits       168729   168735       +6     
- Misses      82948    82965      +17     

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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 it would be safer to "prefer" not to promote a REPLICA performing backup vs wholly exclude it, the reason is I feel promoting a tablet that is backing-up to PRIMARY (if no other candidates are available) is better than doing nothing - which is a 100% outage to writes

If we are down to 1 candidate, still being able to write feels better to me, even if the node is busy with backups as well

[GrahamCampbell]

Another approach could be to cancel the backup in order to enable promotion, if having backups delayed is preferable to being unavailable for writes for a long time.