Grafana: monitor Postgresql instances

[benoit74]

We should add monitoring of Postgresql instances:

• data volume
• locks held for too long (see https://github.com/openzim/zimfarm/issues/826)
• many other metrics / logs which would be helpful to monitor a productive DB

There is a Grafana integration for Postgresql, to be investigated if it match the need

[rgaudin]

A a general rule, I'd like us to carefully considerate every addition. It should be very clear:

• what resource will be consumed to produce said data
• who will use this data and when
• whether this is needed all the time or if that's something we want to enable under certain circonstances.

It's a known failure pattern to slowly, build-up monitoring data that silently increase resources need ($) and slow down applications.