Find node_exporter package at: http://pkg.freebsd.org/FreeBSD:11:amd64/latest/All/

At the time of writing this post: http://pkg.freebsd.org/FreeBSD:11:amd64/latest/All/node_exporter-0.18.1.txz

SSH to PfSense

pkg add http://pkg.freebsd.org/FreeBSD:11:amd64/latest/All/node_exporter-0.18.1.txz
rehash
service node_exporter onestart

EDIT:
How to start node_exporter in PfSense at boot:
https://s55ma.radioamater.si/2019/09/08/pfsense-stuck-at-boot-at-configuring-firewall/

What is node_exporter?
https://prometheus.io/docs/guides/node-exporter/