server,metrics: add a metrics for internal session count

[tiancaiamao]

What problem does this PR solve?

Issue Number: ref #56271

Problem Summary:

What changed and how does it work?

If we get some internal session leak, this new added panel can catch it.

Check List

Tests

• Unit test

• Integration test

• Manual test (add detailed scripts or steps below)

• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[tiprow]

Hi @tiancaiamao. Thanks for your PR.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.9415%. Comparing base (05cff08) to head (74435b3).
Report is 285 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #56272        +/-   ##
================================================
+ Coverage   73.3233%   75.9415%   +2.6182%     
================================================
  Files          1604       1605         +1     
  Lines        444617     452710      +8093     
================================================
+ Hits         326008     343795     +17787     
+ Misses        98674      88597     -10077     
- Partials      19935      20318       +383     

Flag	Coverage Δ
integration	49.3917% <100.0000%> (?)
unit	73.0050% <100.0000%> (+0.4883%)	⬆️

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

Components	Coverage Δ
dumpling	52.9478% <ø> (ø)
parser	∅ <ø> (∅)
br	52.9158% <ø> (+7.6389%)	⬆️

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hawkingrei, lance6716

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-09-25 10:47:33.582615948 +0000 UTC m=+1649323.323039882: ☑️ agreed by hawkingrei.
• 2024-09-25 13:39:20.214430708 +0000 UTC m=+1659629.954854659: ☑️ agreed by lance6716.

[tiprow]

@tiancaiamao: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
fast_test_tiprow	74435b3	link	true	/test fast_test_tiprow

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #56343.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #57034.