Building out an InfluxDB host for metrics collection is pretty straightforward, even piping things into it from various sources isn’t difficult, thanks to the multitude of plugins available out of the box, including some very handy SNMP gathering that I’ll likely go into later when the mood strikes and I have more time. I did exactly this at the day gig not long ago to replace the bulky, somewhat cumbersome check_mk based monitoring I initially set up when I started here (they had nothing in place, except for a few broken zabbix clients, no collection point).

Update Oct 20, 2017 As pointed out in the comments below, most of this is no longer needed since the official release of the telegraf package for pfSense 2.4 and above. I'm still leaving it up for posterity. If any of the pfSense folks read this, some extra configuration options on the settings page for the plugin would be nice to see; but otherwise works like a charm. If you’re like me, a sexy looking dashboard is a difficult thing to look away from.