additional prometheus/grafana exporters/dashboards

our munin replacement is not entirely complete, as there are key parts of the infrastructure that are not monitored. here's a short inventory of what I found in legacy/trac#29681 (moved):

email servers monitoring (eugeni, etc? postfix)

• in debian, possible dashboard
• another approach: email delivery tests

mailman monitoring

no known exporter or dashboard

databases

• postgres exporter, in debian, no offocial dashboard, but many possible dashboards
• mysqld exporter, in debian - possible dashboard another from percona, not officially documented
• generic sql exporter, in debian - credativ config and dashboard

DNS / bind

• in debian, official dashboard

GitLab

there is builtin support for prometheus that has to be configured

those are the other missing things I found during the audit performed while removing Munin:

• spamassassin: ham/spam/total counts, looks for spamd: ((processing|checking) message|identified spam|clean message) in mail.log, could be replaced with ​mtail
• postgres-wal-traffic_: should be covered by the postgres_exporter mentioned above, otherwise hook psql -p "$port" --no-align --command 'SELECT * FROM pg_current_xlog_insert_location()' --tuples-only --quiet | tr -d /, into the node_exporter
• ksm stats: extra memory statistics, might not be very important
• haproxy: https://github.com/prometheus/haproxy_exporter
• per VM disk usage: see legacy/trac#29816 (moved)
• vsftpd: custom mtail plugin, no known exporter or dashboard

See the full review in legacy/trac#29682 (moved) for details on those.

There were also demands from other teams for monitoring, see legacy/trac#29863 (moved) and legacy/trac#30006 (moved) for now.