Monitor usage of Mattermost

Now, we don't have any data on our users using Mattermost, nor if they use Mattermost Slash commands

Sadly, we can't include the service by just counting the table as Services where auto created for a long time, and this table is likely 20 times larger than the projects' table.

However, we could try and leverage omnibus-gitlab!1165 (merged) but this data point is limited to Mattermost being installed on the same server, so not completely accurate.

/cc @markpundsack