sysvar: add 2 sysvars (#10662)

[ti-chi-bot]

This is an automated cherry-pick of #10662

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

• This PR is translated from: Add the description of sysvar max_allowed_packet docs-cn#11328 update some system variable docs-cn#11375 system-variable.md: add description for tidb_simplified_metrics docs-cn#11431 update system variable docs-cn#11505
• Other reference link(s):

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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[TomShawn]

/merge

[ti-chi-bot]

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

Details

Commit hash: 59de5b0

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #10710.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #10711.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #10712.