cloud: align Starter and Essential metrics tabs with console

[Oreoxmt]

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What is changed, added or deleted? (Required)

Update built-in monitoring docs to show Starter and Essential metrics under
Instance Overview and Database Status, and scope Dedicated-only metrics to the
Dedicated plan.

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[gemini-code-assist]

Code Review

This pull request updates the built-in monitoring documentation by wrapping plan-specific metrics sections in <CustomContent> tags for dedicated and starter/essential plans. It also renames "Cluster Status" to "Instance Overview" for starter and essential instances. A review comment suggests changing the heading "Instance Overview" to sentence case ("Instance overview") to comply with the repository style guide.

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-05-29 08:05:36.261102076 +0000 UTC m=+6476.635911772: ☑️ agreed by qiancai.