dm: add replication lag tuning guidance

[joechenrh]

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[gemini-code-assist]

Code Review

This pull request enhances the DM performance troubleshooting documentation by adding a new section on locating bottlenecks through metric analysis and providing detailed steps for diagnosing downstream execution issues in TiDB or TiKV clusters. The review feedback recommends consistent formatting by enclosing technical metric names in backticks to align with the style guide.

[gemini-code-assist]

For consistency and improved readability, please enclose metric names in backticks. This aligns with the formatting of other metrics in this document and the style guide's recommendation for technical terms.

Suggested change

	- If TiDB query latency or TiDB KV-client backoff is high, the bottleneck might exist in the TiDB SQL layer or in the TiDB-to-TiKV path.
	- If `TiDB query latency` or `TiDB KV-client backoff` is high, the bottleneck might exist in the TiDB SQL layer or in the TiDB-to-TiKV path.

References

1. The style guide recommends using backticks for technical terms like command names, options, and paths to ensure consistency and readability. This principle should be extended to metric names as well for consistency. ^(link)

[gemini-code-assist]

To maintain consistency with the rest of the document, please wrap these metric names in backticks. This improves readability and follows the style guide's conventions for technical terms.

Suggested change

	- If TiKV write path metrics such as write RPC latency, scheduler CPU usage, apply CPU usage, write stall, or PD TSO latency are high, check TiKV or PD before tuning DM.
	- If TiKV write path metrics such as `write RPC latency`, `scheduler CPU usage`, `apply CPU usage`, `write stall`, or `PD TSO latency` are high, check TiKV or PD before tuning DM.

References

1. The style guide recommends using backticks for technical terms like command names, options, and paths to ensure consistency and readability. This principle should be extended to metric names as well for consistency. ^(link)