How to get interface and system stats that are part of the F5-SYSTEM-MIB via REST API?

Did you see this in the iControl REST user guide?

To obtain statistics for a resource, this example queries the /Common/my-Pool object for current statistics.
The response that contains the statistical output appears in the second block.
GET https://192.168.25.42/mgmt/tm/ltm/pool/~Common~my-Pool/stats

So for interface stats for example, you could do this:

curl -k -u "admin:admin" https://10.1.0.213/mgmt/tm/net/interface/stats -H "Content-Type:application/json" -X GET

Other than interface, what do you need? Things like CPU and mem are the the sys section, so

curl -k -u "admin:admin" https://10.1.0.213/mgmt/tm/sys/cpu/stats -H "Content-Type:application/json" -X GET