Add Database Monitoring section to Supabase Cloud dashboard (DataDog#23312)

* Add Database Monitoring section to Supabase Cloud dashboard

Add DBM widgets to the Supabase Cloud overview dashboard including
query performance metrics, active query load, wait events, session
breakdowns, table health, and a recommendations section. Follows
the same patterns used in the RDS PostgreSQL dashboard.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Remove managed_observability tag filter from DBM widgets

Only filter by $project (supabase_project_ref) template variable.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Remove redundant Table Size Breakdown widget

Resources by Table already shows Total Size alongside index scans,
live/dead rows, and seq scans — the separate size breakdown was
duplicating data.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: dalextorres