Add prometheus integration with Mattermost

Definition of done:

• Add a servicemonitor and any other necessary pieces (role, role-binding, etc)
• Test and validate data is going to prometheus

Should be able to use t-rex bootstrap as a starting point: https://repo1.dso.mil/platform-one/private/big-bang/t-rex-bootstrap/-/tree/master/apps/mattermost/base/mattermost-pkg/monitoring/prometheus