*: integrate circuitbreaker for get region calls to PD (#58737)

[ti-chi-bot]

This is an automated cherry-pick of #58737

What problem does this PR solve?

Issue Number: close #58780

Problem Summary:

What changed and how does it work?

Introduce a global session variable to control PD Get region call circuitbreaker: https://github.com/tikv/rfcs/blob/master/text/0115-circuit-breaker.md#system-variables

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.

Introduces `tidb_cb_pd_metadata_error_rate_threshold_pct` global system variable which could be used to enable a circuit breaker for PD calls to retrieve region metadata. When a non-zero value is specified, the GetRegion calls to PD will be throttled automatically if error rate exceeds the specified threshold, so that cluster can recover on its own.

[codecov]

Codecov Report

❌ Patch coverage is 84.21053% with 3 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release-8.5@30107c0). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-8.5     #64970   +/-   ##
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Flag	Coverage Δ
integration	51.0161% <84.2105%> (?)
unit	64.8059% <84.2105%> (?)

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

Components	Coverage Δ
dumpling	52.9278% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	63.7714% <0.0000%> (?)

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[rleungx]

/retest

[okJiang]

/retest

[wuhuizuo]

/hold

wait for #64955 be merged.

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-12-15 04:56:12.772264662 +0000 UTC m=+1448917.586042234: ☑️ agreed by lcwangchao.
• 2025-12-16 03:01:35.88999004 +0000 UTC m=+1528440.703767602: ☑️ agreed by yudongusa.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lcwangchao, okJiang, wjhuang2016, yudongusa

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Details

Needs approval from an approver in each of these files:

• OWNERS [lcwangchao,okJiang,wjhuang2016]
• pkg/domain/OWNERS [wjhuang2016]
• pkg/sessionctx/variable/OWNERS [yudongusa]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[okJiang]

/retest

[tiprow]

@ti-chi-bot: 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	af3c4a6	link	true	/test fast_test_tiprow_for_release

Full PR test history. Your PR dashboard.

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[rleungx]

/hold cancel

[okJiang]

/retest

[okJiang]

/retest