I'd recommend that you start by looking at the developer portal documentation, specifically the "Milli Developer Kit Reference Application" and the "Milli Developer Kit Add New Sensor" pages. Next locate and download the SAPI Users Guide from the document center. Visit the GitHub "Milli5 Sensor API" repo and download the source code. Next upload the reference temperature sensor application to an Adafruit Metro Express board and get the end to end solution to work (directions can be found in the developer portal documentation).
Now start looking a the code:
- sapi_mshield shows how a single sensor can be integrated into the Sensor API and the code you will need to build out to support your sensor
- Look a the Examples directory for other examples of sensor implementations (note that we will be building this out over time)
- If your curious and want to look at the reference CoAP server take a look at the itron_coap_server directory
Next, work up a "Hello, World" code set for your sensor and add it the reference application. Get it working end to end so you can see how the device type maps to the resource name and how you access the sensor device via a CoAP URL.
Rinse and Repeat