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

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[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%)	⬇️

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[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

You only need to trigger /merge once, and if the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

If you have any questions about the PR merge process, please refer to pr process.

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: 2b6280a