Do not merge until v8.5.4: tiflash: add a config named graceful_shutdown_wait_timeout (#21734)

[ti-chi-bot]

This is an automated cherry-pick of #21734

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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 enhances the TiFlash server's shutdown mechanism by introducing a configurable grace period. This allows the server to gracefully complete any in-progress Massively Parallel Processing (MPP) tasks before fully shutting down, thereby improving system stability and data integrity during maintenance or restarts. The change ensures that TiDB instances are aware of a TiFlash server's graceful shutdown state and will not assign new tasks to it.

Highlights

• New Configuration Parameter: Introduced a new configuration parameter, graceful_wait_shutdown_timeout, for TiFlash servers.
• Graceful Shutdown Enhancement: This new parameter allows TiFlash to enter a graceful shutdown period, during which it continues to process existing MPP tasks but stops accepting new ones, ensuring ongoing analytical queries can complete.
• Controlled Server Behavior: The configuration defines a maximum wait time (defaulting to 600 seconds) for tasks to finish before a forced shutdown, and TiDB is prevented from sending new MPP tasks to a server in this state.

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[gemini-code-assist]

Code Review

This pull request adds documentation for the new TiFlash configuration parameter graceful_shutdown_wait_timeout. My review includes suggestions to improve clarity, conciseness, and adherence to the documentation style guide, such as using active voice and consistent formatting for parameter names.