monitoring docker container performance using cadvisor / cgroup, best approach
cAdvisor plugin for collectd would be pretty simple. Can you file an issue on github.com/google/cadvisor and we can help you get one.
Alternatively, you can always hit the cAdvisor rest endpoint for the whole machine or a particular container to pull the data into graphite and push it from a helper process. eg.
/api/v1.3/containers/
In any case, file a feature request and we can help you out with the setup.