metrics: add metrics for active users count

[tiancaiamao]

What problem does this PR solve?

Issue Number: ref #55563

Problem Summary:

What changed and how does it work?

Now privilege reload will load the active users instead of all users.
Add a metrics to see how many active users are there currently.

Check List

Tests

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

Load the tidb.json to grafana and check it works

• 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

[ti-chi-bot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[tiancaiamao]

The change is based on #59487
Wait for that PR to merge first.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 73.4534%. Comparing base (67edd7d) to head (b8d50ab).
Report is 7 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #59533        +/-   ##
================================================
+ Coverage   72.9760%   73.4534%   +0.4773%     
================================================
  Files          1699       1699                
  Lines        469387     470388      +1001     
================================================
+ Hits         342540     345516      +2976     
+ Misses       105739     103841      -1898     
+ Partials      21108      21031        -77     

Flag	Coverage Δ
integration	42.8068% <0.0000%> (?)
unit	72.2672% <0.0000%> (+0.0890%)	⬆️

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

Components	Coverage Δ
dumpling	52.6910% <ø> (ø)
parser	∅ <ø> (∅)
br	45.0712% <ø> (+0.0486%)	⬆️

[tiancaiamao]

/test mysql-test

[tiprow]

@tiancaiamao: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

In response to this:

/test mysql-test

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[bb7133]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bb7133, lcwangchao

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Needs approval from an approver in each of these files:

• OWNERS [bb7133,lcwangchao]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-02-28 05:42:11.955081916 +0000 UTC m=+593679.908240182: ☑️ agreed by bb7133.
• 2025-02-28 08:22:44.307708574 +0000 UTC m=+603312.260866835: ☑️ agreed by lcwangchao.

[tiancaiamao]

/retest

[tiprow]

@tiancaiamao: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

In response to this:

/retest

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 kubernetes-sigs/prow repository.

[tiancaiamao]

/retest

[tiprow]

@tiancaiamao: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

In response to this:

/retest

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 kubernetes-sigs/prow repository.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #59847.
But this PR has conflicts, please resolve them!