system tables: add a note for DATA_LENGTH, INDEX_LENGTH, and TABLE_SIZE

[qiancai]

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Add a note for DATA_LENGTH, INDEX_LENGTH, and TABLE_SIZE to explain they are logical estimates instead of actual physical sizes.

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[dveeden]

@mjonss I assume that partitioning and global index work as expected when it comes to the storage size explained here?

[dveeden]

TIKV_REGION_STATUS seems to depend on Prometheus being available. However this isn't noted on https://docs.pingcap.com/tidb/stable/information-schema-tikv-region-status

The result is that size calculation would fail with this error:

ERROR 1105 (HY000): query metric error: [domain:9009]Prometheus address is not set in PD and etcd

If a custom Prometheus is used then the address might not be configured correctly in pd/etcd. If there is no Prometheus we may want to see if there are any alternatives available

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[qiancai]

Removed the needs-cherry-pick-release-8.4 label because the v8.4 docs have been archived at https://docs-archive.pingcap.com/tidb/v8.4 and will no longer receive new updates.