alert: remove keep_alive_total metric

[CbcWestwolf]

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

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• v8.3 (TiDB 8.3 versions)
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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

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• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-08-02 02:04:02.074701463 +0000 UTC m=+55039.293463067: ☑️ agreed by hfxsd.
• 2024-08-02 07:57:33.025081093 +0000 UTC m=+76250.243842696: ☑️ agreed by qiancai.

[hfxsd]

/approve

[ti-chi-bot]

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #18233.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #18234.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.0: #18235.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #18236.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.2: #18237.