*: 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:

• disksing
• lhy1024

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.69%. Comparing base (8df5f5c) to head (f6e1827).

Additional details and impacted files

@@               Coverage Diff               @@
##           release-7.1    #7840      +/-   ##
===============================================
- Coverage        74.77%   74.69%   -0.09%     
===============================================
  Files              411      411              
  Lines            41719    41724       +5     
===============================================
- Hits             31196    31164      -32     
- Misses            7782     7804      +22     
- Partials          2741     2756      +15     

Flag	Coverage Δ
unittests	74.69% <100.00%> (-0.09%)	⬇️

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[disksing]

/merge

[ti-chi-bot]

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[ti-chi-bot]

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

Commit hash: f6e1827