[+] improve "Global Database Overview" dashboard (#869)

* [+] improve "Global Database Overview" dashboard

- Import a new dashboard version from gitlab.com/postgres-ai/pgwatch2
- add datfrozenxid, postgres_role, archiver_pending_count metrics

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Co-authored-by: Vitaliy Kukharik <37010174+vitabaks@users.noreply.github.com>

Author: pashagolub