Add semi-sync monitor to unblock primaries blocked on semi-sync ACKs

[GuptaManan100]

Description

This PR introduces a new component to the vttablet binary to monitor the semi-sync status of primary vttablets. We've observed cases where a brief network disruption can cause the primary to get stuck indefinitely waiting for semi-sync ACKs. In rare scenarios, this can block reparent operations and render the primary unresponsive. More information can be found in the issues #17709 and #17749.

To address this, the new component continuously monitors the semi-sync status. If the primary becomes stuck on semi-sync ACKs, it generates writes to unblock it. If this fails, VTOrc is notified of the issue and initiates an emergency reparent operation.

A metric for the number of outstanding writes from the semi-sync monitor has also been added. Unfortunately, it's not easy to reproduce the problem in an end-to-end test since it requires setting port forward rules (on Mac) or iptable changes (on Linux), both of which require sudo access. So I've added a test but that test doesn't run on CI. It can only be run locally and the password for the root user has to entered when prompted for it.

The semi-sync monitor is used such that the users who aren't running semi-sync, will not see the monitor open at all. It will only run when semi-sync on the primary is turned on.

Related Issue(s)

• Fixes Bug Report: PRS getting stuck due to a momentary network glitch #17709
• Fixes Bug Report: A dropped semi-sync ACK can lead to a primary tablet being indefinitely stuck #17749

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 82.13058% with 52 lines in your changes missing coverage. Please review.

Project coverage is 67.47%. Comparing base (67d081a) to head (4373407).
Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
.../vttablet/tabletmanager/semisyncmonitor/monitor.go	89.65%	24 Missing ⚠️
go/vt/vttablet/tabletserver/tabletenv/config.go	44.00%	14 Missing ⚠️
go/vt/vttablet/tabletmanager/rpc_replication.go	15.38%	11 Missing ⚠️
go/vt/vtorc/inst/instance_dao.go	75.00%	1 Missing ⚠️
go/vt/vtorc/logic/topology_recovery.go	0.00%	1 Missing ⚠️
go/vt/wrangler/testlib/fake_tablet.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17763      +/-   ##
==========================================
+ Coverage   67.41%   67.47%   +0.06%     
==========================================
  Files        1592     1593       +1     
  Lines      258024   258498     +474     
==========================================
+ Hits       173948   174430     +482     
+ Misses      84076    84068       -8     

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[mattlord]

This is looking good! Just had some questions/comments. Let me know what you think.

[mattlord]

LGTM! I only had one minor suggestion. Nice work on this, @GuptaManan100 !

[GuptaManan100]

Done, I've made that change too! 💕