Add circuit breaker variable (#20129)

[ti-chi-bot]

This is an automated cherry-pick of #20129

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• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
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What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s): Implement Circuit Breaker Pattern to Protect PD Leader tikv/pd#8678, Support setting circuit breaker to avoid PD overloaded tidb#58780

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• Delete files
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[gemini-code-assist]

Summary of Changes

Hello @ti-chi-bot, I'm Gemini Code Assist¹! 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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[gemini-code-assist]

Code Review

This pull request adds documentation for the new tidb_cb_pd_metadata_error_rate_threshold_ratio system variable. My review focuses on improving the clarity and formatting of the description to align with the repository's style guide. I've suggested a minor wording change and added backticks to numerical values for consistency.

[ti-chi-bot]

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