PMM-12961 [Tech Preview] New Databases Overview Dashboard (#1571)

* Create databases_overview.json

* Update databases_overview.json

clean up from @YashSartanpara1

* PMM-7 fix clickhouse datasource for db overview

* Update dashboards/Experimental/databases_overview.json

Co-authored-by: Alex Demidoff <[email protected]>

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Co-authored-by: Roman Novikov <[email protected]>
Co-authored-by: Yash Sartanpara <[email protected]>
Co-authored-by: Alex Demidoff <[email protected]>