Add support for attaching aggregation labels to sysctl objects.

I'm currently working on writing a metrics exporter for the Prometheus
monitoring system to provide access to sysctl metrics. Prometheus and
sysctl have some structural differences:

• sysctl is a tree of string component names.
• Prometheus uses a flat namespace for its metrics, but allows you to attach labels with values to them, so that you can do aggregation.

An initial version of my exporter simply translated

hw.acpi.thermal.tz1.temperature

to

sysctl_hw_acpi_thermal_tz1_temperature_celcius

while we should ideally have

sysctl_hw_acpi_thermal_temperature_celcius{thermal_zone="tz1"}

allowing you to graph all thermal zones on a system in one go.

The change presented in this commit adds support for accomplishing this,
by providing the ability to attach labels to nodes. In the example I
gave above, the label "thermal_zone" would be attached to "tz1". As this
is a feature that will only be used very rarely, I decided to not change
the KPI too aggressively.

Discussed on: hackers@
Reviewed by: cem
Differential Revision: https://reviews.freebsd.org/D8775