Monitors interface and how to deal with non reactive interfaces

Hey folks, I was trying fix somethings in the nrpe and nagios charm and evaluate how I could use the monitors interface myself in another charm, but I quickly found out that there is no monitors interface to be found in the layer-index. Searching online, I found a rather old bug asking for some documentation on the interface.

How do I interact/modify/use the interface in a reactive charm?