Mattermost grafana dashboard

Description

Adding grafana dashboard json file and configmap to package when monitoring.enabled: true and enterprise.enebled: true (enterprise required for metrics scraping).

ServiceMonitor was already present and healthy in prometheus.

Testing notes

The dashboard does not default to server pointing at the mattermost instance. Rather this has to be selected manually (to which data is being accurately reported).

Issues

Relates to https://repo1.dso.mil/platform-one/big-bang/bigbang/-/issues/1214