metrics: rename mergeCheckerSpecialPeerCounter to mergeCheckerUnhealthyRegionCounter

[bufferflies]

What problem does this PR solve?

Issue Number: Ref #4399

What is changed and how does it work?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Code changes

Side effects

Related changes

Release note

[None.](metrics: rename `mergeCheckerSpecialPeerCounter` to `mergeCheckerUnhealthyRegionCounter`)

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• CabinfeverB
• lhy1024

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[lhy1024]

/merge

[ti-chi-bot]

@lhy1024: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 98b58c8

[ti-chi-bot]

@bufferflies: Your PR was out of date, I have automatically updated it for you.

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[codecov]

Codecov Report

Merging #7117 (d9d3d2c) into master (0888ef6) will increase coverage by 0.01%.
Report is 6 commits behind head on master.
The diff coverage is 59.30%.

@@            Coverage Diff             @@
##           master    #7117      +/-   ##
==========================================
+ Coverage   74.33%   74.34%   +0.01%     
==========================================
  Files         437      440       +3     
  Lines       46855    47150     +295     
==========================================
+ Hits        34831    35056     +225     
- Misses       8947     9005      +58     
- Partials     3077     3089      +12     

Flag	Coverage Δ
unittests	74.34% <59.30%> (+0.01%)	⬆️

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