infoschema,metrics: add a metric for infoschema v2 cache objects count

[tiancaiamao]

What problem does this PR solve?

Issue Number: ref #50959

Problem Summary:

What changed and how does it work?

Add a metrics for infoschema v2 schema cache objects count.
So that the user can calculate or estimate the size of each table.

Check List

Tests

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

Use the new tidb.json

• 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 75.1078%. Comparing base (5301602) to head (8aaafb4).
Report is 25 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #58781        +/-   ##
================================================
+ Coverage   73.1389%   75.1078%   +1.9688%     
================================================
  Files          1677       1723        +46     
  Lines        463899     474592     +10693     
================================================
+ Hits         339291     356456     +17165     
+ Misses       103747      95886      -7861     
- Partials      20861      22250      +1389     

Flag	Coverage Δ
integration	48.8750% <62.5000%> (?)
unit	72.4962% <100.0000%> (+0.1647%)	⬆️

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	61.6877% <ø> (+15.9397%)	⬆️

[GMHDBJD]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: GMHDBJD, tangenta

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Needs approval from an approver in each of these files:

• OWNERS [GMHDBJD,tangenta]
• pkg/infoschema/OWNERS [GMHDBJD,tangenta]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-01-09 08:22:22.69298678 +0000 UTC m=+428285.981818486: ☑️ agreed by tangenta.
• 2025-01-14 04:07:32.355417288 +0000 UTC m=+5123.810463436: ☑️ agreed by GMHDBJD.

[tiancaiamao]

/test pull-lightning-integration-test

[tiprow]

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In response to this:

/test pull-lightning-integration-test

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