Online DDL: consider UUID throttle ratios in user_throttle_ratio

[shlomi-noach]

Description

See #17849 for details.
The online DDL executor checks the throttle ration for each migration;'s UUID and updates user_throttle_ratio with the hihest ratio of (uuid-ratio, online-ddl ratio).

Related Issue(s)

Fixes #17849

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

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 35.00000% with 13 lines in your changes missing coverage. Please review.

Project coverage is 67.46%. Comparing base (413b953) to head (91d7089).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vttablet/onlineddl/executor.go	0.00%	11 Missing ⚠️
...ttablet/tabletserver/throttle/base/app_throttle.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17850      +/-   ##
==========================================
+ Coverage   67.44%   67.46%   +0.01%     
==========================================
  Files        1592     1592              
  Lines      258205   258222      +17     
==========================================
+ Hits       174151   174211      +60     
+ Misses      84054    84011      -43     

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