Add labels to sysctls related to clocks.

Sysctls like kern.eventtimer.et.*.quality currently embed the name of
the clock device. This is problematic for the Prometheus metrics
exporter for two reasons:

• Some of those clocks have dashes in their names, which Prometheus doesn't allow to be used in metric names.
• It doesn't allow for extracting the same property of all clocks on the system from within a single query.

Attach these nodes to have a label, so that the Prometheus metrics
exporter gives these metric a uniform name with the name of the clock
attached as a label.

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