Introduced in 9.11, there is the /api/cluster/counter/tables endpoint. This provides access to many different objects and counters for the system. Specifically, you can look at the /api/cluster/counter/tables/volume and/or /api/cluster/counter/tables/qos_detail to see related counters.
There will be a document published that lists all of the available counters once the GA build for 9.11 is published. A link to it will be added to this page: https://devnet.netapp.com/restapi.php