Expose metrics for all available databases on a PMM monitored PostgreSQL server

User story:
When I monitoring PG server I need to get metrics about all databases on the server so that I can better understand the performance impact created by individual databases

UI/UX:
N/A

Acceptance criteria

• We have metrics collected regarding all databases accessible with the passed user
• Current dashboards working as expected

Out of scope:

• Any specific dashboards will be developed later separately
• Changes in instructions to create a user permissions

Suggested implementation:
TBD

How to test:

• check that exporter has next flags:
  --auto-discover-databases
  --exclude-databases=template0,template1,postgres
• place file queries-postgresql-user-table.yml with a custom queries into /usr/local/percona/pmm2/collectors/custom-queries/postgresql/medium-resolution/ folder
• restart exporter
• check that metric pg_stat_user_tables_analyze_count collected for all existed databases (except template0,template1,postgres)
  

Details:
TBD