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.