After your first version has been released, you will need to prepare the next version and the one after that. This requires you to set up a kind of feedback loop to know what to improve next. This is the Monitor phase.
To get accurate data in your feedback loop, you will have to monitor various aspects of your APIs. This ranges from developer engagement to API health, to API usage, and so on. The “Time to first Hello, World!” is a useful metric to track to know if your APIs are easy enough to be learned quickly by new developers.