Let all FEATURE()s use the same Prometheus metric.

Without this change, every individual FEATURE() declaration would have
an individual metric in Prometheus. Though this wouldn't be harmful, it
would look very cluttered.

By letting it use a single metric with the name of the feature attached
as a label, it also becomes easier to search, as you can apply regex
matching, etc.

Reviewed by: cem
Differential Revision: https://reviews.freebsd.org/D8775