There are the docs:
https://docs.mender.io/add-ons/monitor
Which shows you how to use the currently supported monitoring subsystems:
- log
- services
- dbus
However, it does not seem we have any good tutorials for writing custom subsystems present (this should be amended imo):
In the meantime, I am actually writing some demo code that we are about to ship in our demo setup, which will be some variation of this Pr.
Here you can see how I have created a docker events subsystem, and added a log-monitor for USB disconnects.
Have a look at this, and tell me if it is helpful or not. I will help you out here, then you can help me out improving our documentation on the subject for future users 