Add net-mgmt/metronome:

Mini-graphite that uses client-side java script to render graphs w/o
depending on graphite.

metronome implements the carbon protocol, so anything that can feed
Graphite can feed metronome. We also accept submissions via POST.

WWW: https://github.com/ahupowerdns/metronome

Approved by: mat (mentor)
Differential Revision: https://reviews.freebsd.org/D13285