ADD usage of PG_stat_monitor Source in Add Remote PG_stat-monitor

The Goal: we need to let the user use pg_stat_monitor source when adding remote PG  server 

Proposed solution: add an additional checkbox on  https://pmmdemo.percona.com/graph/d/pmm-add-instance/pmm-add-instance?   for PG Stat Monitor

UI:

an additonal line with checkbox, not selected by default : "Use PG Stat Monitor"

update: 
the user will have 3 options, so let's put them as for now as  radiobutton: 

Query metrics source:

• none (default)
• PG stat statement
• PG stat monitor