Attach a "thermal_zone" label to the ACPI thermal zone sysctls.

In order to make Prometheus do graphing/alerting on thermal sensors in a
generic fashion, we should attach the name of the thermal zone device as
a label. That way there is only a single metric for the temperature of a
thermal zone, with its name attached as a label.

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