Implement ActiveRecord level monitoring

Implement ActiveRecord level monitoring for the following:
(This has already been implemented in zync: check why operations did not add to dashboard)

• Size of connection pool
• Number of connections in the connection pool
• Number of waiting in the queue of the connection pool
• Time waiting in the queue

Please read the description of the epic this task belongs to.

See for reference: https://github.com/3scale/zync/blob/master/lib/prometheus/active_record.rb#L7

A prrerequisite for the work above is already done (as part of this issue) in https://github.com/3scale/porta/pull/2206 - added stat method, which is missing in Rails 5.0