Tablet throttler: remove cached metric associated with removed tablet

[shlomi-noach]

Description

Bugfix. See #16554 for context.

Related Issue(s)

Fixes #16554

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
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.77%. Comparing base (0b7c0e4) to head (ad4d87e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16555      +/-   ##
==========================================
- Coverage   68.78%   68.77%   -0.01%     
==========================================
  Files        1556     1556              
  Lines      199803   199809       +6     
==========================================
- Hits       137429   137424       -5     
- Misses      62374    62385      +11     

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[mattlord]

@shlomi-noach Looks like there's now a few relevant unit tests that are flaky (and some data races). Let me know when it's all good and I'll review.

[shlomi-noach]

@mattlord tests + race are all fixed.