Add circuit breaker variable (#20129)

[ti-chi-bot]

This is an automated cherry-pick of #20129

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• [ ] I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

What is changed, added or deleted? (Required)

Which TiDB version(s) do your changes apply to? (Required)

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For details, see tips for choosing the affected versions.

• [x] master (the latest development version)
• [x] v9.0 (TiDB 9.0 versions)
• [ ] v8.5 (TiDB 8.5 versions)
• [ ] v8.4 (TiDB 8.4 versions)
• [ ] v8.3 (TiDB 8.3 versions)
• [ ] v8.1 (TiDB 8.1 versions)
• [ ] v7.5 (TiDB 7.5 versions)
• [ ] v7.1 (TiDB 7.1 versions)
• [ ] v6.5 (TiDB 6.5 versions)
• [ ] v6.1 (TiDB 6.1 versions)
• [ ] v5.4 (TiDB 5.4 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s): https://github.com/tikv/pd/issues/8678, https://github.com/pingcap/tidb/issues/58780

Do your changes match any of the following descriptions?

• [ ] Delete files
• [ ] Change aliases
• [ ] Need modification after applied to another branch
• [ ] Might cause conflicts after applied to another branch

[gemini-code-assist[bot]]

Summary of Changes

Hello @ti-chi-bot, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a new system variable, tidb_cb_pd_metadata_error_rate_threshold_ratio, to enhance TiDB's resilience. This variable enables a configurable circuit breaker mechanism that monitors the error rate of requests to the Placement Driver (PD), allowing the system to proactively respond to PD-related issues and prevent potential cascading failures.

Highlights

• New System Variable: Introduces tidb_cb_pd_metadata_error_rate_threshold_ratio, a new system variable in TiDB v9.0.0.
• Circuit Breaker Control: This variable allows users to configure a circuit breaker that triggers based on the error rate of specific requests sent to the Placement Driver (PD).
• Configuration: Setting the variable to 0 disables the circuit breaker, while a value between 0.01 and 1 enables it, defining the error rate threshold for activation.

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[APPROVALNOTIFIER] This PR is NOT APPROVED

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