Online DDL metrics: OnlineDDLStaleMigrationMinutes

[shlomi-noach]

Description

This PR introduces a new stats gauge for Online DDL, OnlineDDLStaleMigrationMinutes.
Vitess (vreplication based) migrations that are found running but show no vreplication liveness in the past 5 seconds are logged, and the metric OnlineDDLStaleMigrationMinutes is updates as the highest number of staleness minutes among them.
When all running migrations have liveness within the 5min range, or if there's no running migrations, then OnlineDDLStaleMigrationMinutes is set to zero.

Related Issue(s)

Fixes #18416

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
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• Documentation was added or is not required

Deployment Notes

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 38 lines in your changes missing coverage. Please review.

Project coverage is 67.49%. Comparing base (efae039) to head (34db5a6).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vttablet/onlineddl/executor.go	0.00%	38 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18417      +/-   ##
==========================================
- Coverage   67.51%   67.49%   -0.02%     
==========================================
  Files        1607     1607              
  Lines      262687   262741      +54     
==========================================
- Hits       177340   177327      -13     
- Misses      85347    85414      +67     

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