Technical preview of new PostgreSQL extension 'pg_stat_monitor'

As part of implementing pg_stat_monitor we should extend management pmm-managed API.

How to test it:

1. Add new postgresql service with query source equal to pg_stat_monitor with swagger.
   1. No error occured
   2. Information about created service and pg_stat_monitor agent is returned.
2. Send Get request with recently added agent_id.
   1. No error occured
   2. Information about requested pg_stat_monitor agent is returned.
3. Send List agents request in inventory API with swagger.
   1. No error occured
   2. New pg_stat_monitor agent in the list.

Test new metrics

1. Setup percona distribution of PG 11 or percona distribution of PG 12 with installed pg_stat_monitor
   • Try with pg_stat_monitor.pgsm_normalized_query enabled
     • Check that we have only fingerprint
   • Try with pg_stat_monitor.pgsm_normalized_query disabled and disabled query examples
     • Check that we have only fingerprint
   • Try with pg_stat_monitor.pgsm_normalized_query disabled and enabled query examples
     • Check that we have fingerpint and example.
2. Check that we have the same metrics which is available in pg_stat_statements.
3. Check that we have new metrics
   • cpu_user_time
   • cpu_sys_time

If some metrics has wrong values then it is OK, because ticket was already created for that.

If flag will be unavalaible please use them without prefix "pgsm_". As you can see here:
https://github.com/percona/pg_stat_monitor flag for max query length is pg_stat_monitor.pgsm_query_max_len, but in my PR worked only pg_stat_monitor.query_max_len (https://github.com/percona/pmm-agent/pull/183/files#diff-4e5e90c6228fd48698d074241c2ba760R67).

What should be done:

• PMM-managed
  • New agent type in database
  • New agent type in List and Get endpoints of inventory API
  • New query source in Management API
  • Skip generating  prometheus scrape config for new agent type
  • Generate postgresql DSN for new agent type.
  • Send request to pmm-agent to run new agent
  • Forward new metrics from pmm-agent to qan-api
• PMM-agent
  • Run pg_stat_monitor agent
• PMM API tests
  • Extend tests for list and get endpoints
  • Extend tests for management API