Did create some Munin plugins and wrote about it:

https://www.philipp.haussleiter.de/2014/06/writing-your-own-munin-plugins-pt1-overview/
https://www.philipp.haussleiter.de/2014/06/writing-your-own-munin-plugins-pt2-counting-vpnd-connections-on-mac-os/
https://www.philipp.haussleiter.de/2014/12/writing-munin-plugins-pt3-some-stats-about-vmware-fusion/