Monitoring user

## Steps to reproduce

There were no PostgreSQL metrics so I saw these errors in one of the units:

```
Jun 17 20:05:32 juju-007f82-prod-chat-synapse-db-42 charmed-postgresql.prometheus-postgres-exporter[1392284]: ts=2024-06-17T20:05:32.276Z caller=collector.go:194 level=error msg="collector failed" name=database duration_seconds=0.006477263 err="pq: password authentication failed for user \"monitoring\""
```

## Expected behavior

Grafana dashboard with all metrics.

## Actual behavior
Only OS metrics.

## Versions

Operating system:

Juju CLI: 3.5.1

Juju agent: 3.1.8

Charm revision: 14/edge 405

LXD:

## Log output

Juju debug log:

## Additional context
I was able to fix it by setting the monitoring user password again. (Canonical-only) I wrote a how to:
https://discourse.canonical.com/t/how-to-fix-postgresql-charm-not-sending-metrics/3801

Maybe it could be considered adding an action to do this.