coordinator: use a healthy region count to start coordinator (#7044)

[ti-chi-bot]

This is an automated cherry-pick of #7044

What problem does this PR solve?

Issue Number: Close #6988 ,Close #7016

What is changed and how does it work?

• different source of region, regard FromSync and FromStorage as the stale region
• add a stale region count in regionTree to start coordinator first.
• atomic add/sub stale regions number by region updated

Check List

Tests

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

Release note

use a healthy region count to avoid starting coordinator timeout.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• JmPotato
• rleungx

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Details

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[HuSharp]

/retest

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (85f1525) 74.91% compared to head (2b6280a) 74.89%.

Files	Patch %	Lines
server/cluster/cluster.go	0.00%	0 Missing and 1 partial ⚠️
server/region_syncer/client.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           release-7.1    #7364      +/-   ##
===============================================
- Coverage        74.91%   74.89%   -0.02%     
===============================================
  Files              409      409              
  Lines            41332    41354      +22     
===============================================
+ Hits             30962    30973      +11     
- Misses            7661     7672      +11     
  Partials          2709     2709              

Flag	Coverage Δ
unittests	74.89% <94.44%> (-0.02%)	⬇️

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[HuSharp]

@JmPotato @rleungx can you leave a merge lable? thx! :)

[JmPotato]

/merge

[ti-chi-bot]

@JmPotato: 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.

Details

Commit hash: 2b6280a