Only initialise metrics for the most common feature categories
Most feature categories aren't used at all yet, even on GitLab.com. This
meant we generated a huge amount of series that would never change. As a
compromise between initialising nothing (and having gaps, plus
unpredictable memory usage in Prometheus) and initialising
everything (explosion in number of series), we only initialise for the
top five categories as of today, plus the default category of unknown.
The top five categories do include not_owned, but that's probably OK.
This initialisation was added in !45755 (merged).
Top five categories from https://log.gprd.gitlab.net/goto/3af22ed4d2c7be5515e99d0068c12ea6, which is slightly different to the categories in Prometheus, but probably not enough to matter: gitlab-com/gl-infra/scalability#643 (comment 439020784)