Those two values are contained within the "/storage/volumes/{uuid}" API, specifically the "files" property. A curl command to output that data for a given volume UUID would look like this:
curl -X GET "https://<cluster name/IP>/api/storage/volumes/{your_volume_uuid}?fields=files" -H "accept: application/json"
{ "uuid": "320a72dd-bd11-11e9-876d-000c29fdc162", "name": "test302_data", "files": { "maximum": 7782389, "used": 96 } }
For reference (and because the new REST API documentation is awesome), you can look at the entire API documentation by going to "https://<cluster name/IP>/docs/api", which will allow you to interact with the API to try it out before writing code that utilizes it. You can also view the raw Swagger YAML file from the screenshot below:
I just did a "Ctrl+F" and searched for a couple key words from your post from the ZAPI documentation, and was able to find out the proper API call from there. Hope that helps!
Donny
My apologies, reading comprehension isn't my strong suit apparently. 
I do not see a way to collect this data at the aggregate level from the REST API.
