Are you looking for the usage of the Kubelet, the whole machine, or a specific container/pod?
For Kubelet: today we won't be able to since it doesn't run in a container (there are plans for this to be the case in the near future).
For the whole machine: You are looking for the root container ("/") in cAdvisor.
For a container/pod: You can query the individual container in cAdvisor.
If you're looking for the usage of all non-containerized things on the machine there is no API for that today but you can do:
usage at root - (sum of usage of all children of root)