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.
Nowadays (since release 20.1 and up), munin is available as a package to OPNsense itself, so no reason to temporarily use the FreeBSD repo any longer (this comes with security concerns as well since no updates are provided).