*: fix deletion of region stats metrics after resign

[iosmanthus]

What problem does this PR solve?

Issue Number: Close #6366

What is changed and how does it work?

https://github.com/prometheus/client_golang/blob/d7896d4bd082b17e525c29055d79cc29484aa9cb/prometheus/vec.go#L125 doesn't reset the gauge in this metrics vec, instead, it deletes them all.

*: fix deletion of region stats metrics after resign

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has the configuration change
• Has HTTP API interfaces changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• rleungx

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[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.03 🎉

Comparison is base (1b75108) 75.08% compared to head (1b87793) 75.11%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6367      +/-   ##
==========================================
+ Coverage   75.08%   75.11%   +0.03%     
==========================================
  Files         408      408              
  Lines       40598    40611      +13     
==========================================
+ Hits        30481    30505      +24     
+ Misses       7476     7467       -9     
+ Partials     2641     2639       -2     

Flag	Coverage Δ
unittests	75.11% <100.00%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/statistics/region_collection.go	97.67% <100.00%> (+0.14%)	⬆️

... and 27 files with indirect coverage changes

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[rleungx]

/check-issue-triage-complete

[rleungx]

/merge

[ti-chi-bot]

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

/run-all-tests

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.

[ti-chi-bot]

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

Commit hash: 1b87793

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #6368.