status-variables: Add conditional status that are not available with unistore

[dveeden]

What is changed, added or deleted? (Required)

When running TiDB with tiup playground, tiup cluster, tidb-operator or other situations where it runs with at least PD, TiKV and TiDB there are more status variables available than when running with only TiDB (./bin/tidb-server, unistore). This adds the missing variables.

See also pingcap/tidb#22286

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[dveeden]

1. We could add something like "not available with unistore" or "only available with TiKV", but I don't think we should.
2. These GC status vars don't seem to be mentioned in the GC docs. We could consider to add a link from the GC docs back to the status var docs.

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-01-23 08:47:55.510396225 +0000 UTC m=+864117.074693925: ☑️ agreed by ran-huang.
• 2024-02-29 10:38:43.097185481 +0000 UTC m=+1131211.844808591: ☑️ agreed by qiancai.

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[qiancai]

/approve

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[qiancai]

/retest

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #16650.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.6: #16651.