Catch long running tasks in DB by exposing metrics from `pg_stat_activity`

[Harkishen-Singh]

We should add metrics from pg_stat_activity such that, this metric promscale_sql_database_pg_stat_activity{backend_type="", query="", pid="", application_name=""} will have 2 value, 0 or 1. If an entry exists in the database, then 1, otherwise 0.

This metric will be very useful in debugging since it catches long-running services in the database (the graph will show a long line steady at 1 for several hours/days), which could potentially be the cause of CPU load.

We may have to do some changes in the database metrics engine to add support for this.