*: 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:

• CabinfeverB
• JmPotato

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[codecov]

Codecov Report

Attention: Patch coverage is 82.60870% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 75.46%. Comparing base (1467007) to head (3ce032a).

Files	Patch %	Lines
server/core/region.go	50.00%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           release-6.5    #7976      +/-   ##
===============================================
- Coverage        75.63%   75.46%   -0.18%     
===============================================
  Files              332      332              
  Lines            33784    33790       +6     
===============================================
- Hits             25553    25499      -54     
- Misses            6033     6080      +47     
- Partials          2198     2211      +13     

Flag	Coverage Δ
unittests	75.46% <82.60%> (-0.18%)	⬇️

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[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

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[ti-chi-bot]

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

Commit hash: 3ce032a