Support specifying expected primary in ERS/PRS

[timvaillancourt]

Description

This PR allows a EmergencyReparentShard/PlannedReparentShard request to contain the alias of the primary we expect to be the current primary for the action to succeed. If this alias is incorrect or stale, the reparent fails with an error indicating there is a failed precondition/mismatch.

This is useful to prevent races between external automation that runs reparents and Vitess (vtorc and vtctld), which may reparent a shard in the time external automation is creating/processing it's own reparent request. With this support added, automation with a stale view of the world will encounter an error instead of triggering an potentially unnecessary reparent

In the future, Vitess could use this field internally to be more explicit but because everything uses shard locks, it should never encounter a mismatch

Related Issue(s)

Resolves #16430

Checklist

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Deployment Notes

[vitess-bot]

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[timvaillancourt]

I think I prefer the field-name RequiredPrimary now but let's vote on it

🎉 for ExpectedPrimary
🚀 for RequiredPrimary
👀 for ExpectedCurrentPrimary
😄 for RequiredCurrentPrimary

Or other suggestions appreciated

[GuptaManan100]

LGTM!

[deepthi]

I think I prefer the field-name RequiredPrimary now but let's vote on it

🎉 for ExpectedPrimary 🚀 for RequiredPrimary

Or other suggestions appreciated

RequiredCurrentPrimary OR ExpectedCurrentPrimary :)

[deepthi]

LGTM. Once the flag name is final, you should edit the PR description to document command usage. I've added a label that prevents merge, and you can remove the label and merge it at that point.

[timvaillancourt]

LGTM. Once the flag name is final, you should edit the PR description to document command usage. I've added a label that prevents merge, and you can remove the label and merge it at that point.

Sounds good! It looks like there weren't many strong opinions on naming

I think I've flip/flopped back to leaving this as-is. My reasoning is if I say what I'm asking to do out loud it is "do X reparent and I expect Y to be still be the primary". And I feel adding Current is redundant because there can only be a single primary