Since OPNsense is more vanilla FreeBSD, it is a lot simpler to install additional FreeBSD software on it than on pfSense.
Temporarily enable the FreeBSD repo by editing /usr/local/etc/pkg/repos/FreeBSD.conf and changing no to yes. Then run
pkg install munin-node
It will take a while to download and install all the necessary perl modules.
Edit: Since OPNsense 20.1 Munin-node is available for installation through the GUI as the os-munin-node plugin. Enabling the base FreeBSD repo is no longer necessary. See this forum topic for a detailed configuration tutorial.
Unless this file already existed, a sample configuration file has been placed as /usr/local/etc/munin/munin-node.conf.
You can edit this file according to your needs. Remember to add an allow rule for your remote system running the munin data scraper:
Once installed, the Munin client will not be started automatically. To enable it on startup, add the following line in /etc/rc.conf:
To start munin-node now, run:
For a list of plugins that will probably work run
If this is a fresh install run
/usr/local/sbin/munin-node-configure --shell | sh -x
to enable the suggested plugins automatically.
Wait 5-10 minutes to see if the node is accessible from the munin scraper and gets added to the systems list.
Finally, remember to disable the FreeBSD repository in OPNsense.