Hi James Eyre,
As it happens I found myself scratching around for similar kinds of resources just the other day. While I didn't come across any how to guides as such, I'd recommend taking a looking that the samples produced by the ArcGIS Monitor team over at https://arcgismonitor.maps.arcgis.com/home/gallery.html
Once you read through a few examples a consistent pattern starts to emerge, whereby the extension is written for node.JS and the index.js exports an execute function that returns a deferred promise to the collector, and an inputs object is exported to define the parameters of execution. In many cases the business logic of the extension is wrapping a powershell script or a compiled binary, but on the javscript side they all follow a pretty similar kind of pattern. In particular I found the File Read Write extension a good one to take a look as it demonstrates the general concept without getting too weighed down in additional complexity.
I haven't gotten around to writing any of my own extensions for Monitor yet, but I see a lot of potential in this space and I'd love to see some more doco and/or hear from anyone that's done this before.
Anyway, hope that at least helps with getting started!
Hi James Eyre,
If you still happen to be looking for info, I've written up a blog post that may be of interest to you https://community.esri.com/people/jbranderesriaustralia-com-au-esridist/blog/2020/09/25/a-crash-cour...