system-variable.md: add description for tidb_simplified_metrics

[tangenta]

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What is changed, added or deleted? (Required)

Which TiDB version(s) do your changes apply to? (Required)

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• master (the latest development version)
• v6.3 (TiDB 6.3 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)
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• v5.1 (TiDB 5.1 versions)
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What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s): metrics: add a config to unregister the unused metrics tidb#34990

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

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn

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After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

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[TomShawn]

LGTM

[TomShawn]

/merge

[ti-chi-bot]

@TomShawn: /merge in this pull request requires 2 approval(s).

Details

In response to this:

/merge

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[TomShawn]

/remove-status LGT1
/status LGT2

[TomShawn]

/merge

[ti-chi-bot]

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

Details

Commit hash: 3ff183e

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #11451.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #11452.