Update built-in-monitoring-premium.md

[huoyao1125]

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[gemini-code-assist]

Code Review

This pull request introduces an unnecessary blank line before the "Database" heading in the built-in monitoring documentation. The reviewer correctly pointed out that only a single blank line is required before headings according to the style guide, and requested the removal of the duplicate blank line.

[gemini-code-assist]

Only a single blank line is required before headings. Please remove the duplicate blank line.

References

1. Add a blank line before and after headings and lists. ^(link)

[huoyao1125]

Suggestion: add two TTL monitoring metrics to the Overview table

Two new TTL metrics are being introduced to the Metrics page (Instance Overview tab) for Premium clusters. I'd like to suggest adding them as two new rows at the end of the Overview table (after Disconnection Count).

Proposed addition:

| Table Count by TTL Schedule Delay | `{name}` | Shows how many TTL-enabled tables fall into each schedule delay range. The `name` label is a delay bucket. Possible bucket values: `01 hour`, `02 hour`, `06 hour`, `12 hour`, `24 hour`, `72 hour`, `one week`, `others`. |
| TTL Insert/Delete Rows by Day | `insert current day`, `delete current day`, `insert last day`, `delete last day`, `insert 2 days ago`, `delete 2 days ago` | Shows the total number of rows written into TTL-tracked tables (insert) and rows deleted by TTL jobs (delete) within a calendar day. A persistent insert/delete imbalance — where inserts consistently exceed deletes — indicates TTL is not keeping pace with data ingestion, leading to storage growth. Day-over-day comparison helps distinguish normal traffic variation from a degradation trend. |

/cc @yoanahuo