alert: remove keep_alive_total metric (#18227)

[ti-chi-bot]

This is an automated cherry-pick of #18227

Since pingcap/tidb#40142, we did not have Keep Alive OPM metric anymore. But we forget to update the doc.

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

• This PR is translated from:
• Other reference link(s): https://asktug.com/t/topic/1029698

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

[hfxsd]

/approve

[ti-chi-bot]

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