This commit adds basic Grafana dashboards for Dovecot and Prosody, to be
automatically deployed alongside the relevant services (if Grafana
itself is enabled).
As the former does not do exactly what it says it does in documentation.
Also, we decrease the default scrape interval for Prometheus from 1m to
30s to improve granularity of data.
This adds basic metric definitions across a number of different events,
to be expanded upon as needed in the future. Several metrics are given
additional vectors with low cardinality.
