*: fix region stats check (#7748)

[ti-chi-bot]

This is an automated cherry-pick of #7748

What problem does this PR solve?

Issue Number: Close #7728

What is changed and how does it work?

1. isNew is useless because region source is inexact when leader transfer and no sync. Now, no need to hold isNew because saveCache must be true if the region in cache does not have leader.
2. check whether the region is observed by its own map

Check List

Tests

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

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]

This pull request has been approved by:

• HuSharp
• 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.

[CabinfeverB]

wait for #7818

[codecov]

Codecov Report

Merging #7812 (04a02a7) into release-7.5 (b8feb2b) will increase coverage by 0.04%.
The diff coverage is 95.83%.

Additional details and impacted files

@@               Coverage Diff               @@
##           release-7.5    #7812      +/-   ##
===============================================
+ Coverage        74.49%   74.53%   +0.04%     
===============================================
  Files              444      444              
  Lines            48136    48140       +4     
===============================================
+ Hits             35857    35883      +26     
+ Misses            9086     9068      -18     
+ Partials          3193     3189       -4     

Flag	Coverage Δ
unittests	74.53% <95.83%> (+0.04%)	⬆️

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

[CabinfeverB]

/merge

[ti-chi-bot]

@CabinfeverB: 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: 04a02a7