Add the doc for the TTL feature

[lcwangchao]

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[REVIEW NOTIFICATION]

This pull request has been approved by:

• SunRunAway
• ran-huang

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[YangKeao]

Rest LGTM

[ran-huang]

Suggested change

	作为实验特性，暂时不兼容包括 BR, Lightning, TiCDC 在内的数据导入导出以及同步工具。
	作为实验特性，暂时不兼容包括 BR、TiDB Lightning、TiCDC 在内的数据导入导出以及同步工具。

[ran-huang]

这里不兼容的意思是指通过工具导入导出的表里 TTL 不生效，还是指开启 TTL 之后无法使用工具导出数据？可以具体说明一下吗？

[lcwangchao]

主要是现在没测过。。所以先这么写了。。

[lcwangchao]

@ran-huang I've updated the doc and added some metrics descriptions, PTAL

[lcwangchao]

@lcwangchao Is the metrics also available on TiDB Cloud? I think we need them also.

The TiDB Cloud exposes little monitoring graph in the console. But for the inner clinic grafana dashboard, it will be included automatically

[ran-huang]

Rest LGTM

[ran-huang]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: ee73b9f