Enable cgroup stats for misc

[mythi]

While the misc built-in controller does not have too many resources to manage today, we are working on adding support for SGX EPC memory to it and for which "per container" stats are very useful.

With OCI / runc, I can already set up misc.max easily through config.json:

"unified": {​
    "misc.max": "sgx_epc 42"​
}

that gives me (in my setup):

cat /sys/fs/cgroup/kubepods.slice/kubepods-besteffort.slice/kubepods-besteffort-pod6df0ca6a_5cf4_4839_82dc_7337d00b71ed.slice/cri-containerd-d7cbd4f59bb80c46b39656bcfad0e3a4c16ec62a3919d5d559f2b29255191437.scope/
misc.max
sgx_epc 42

misc.current returns the per container usage of sgx_epc.

I'm looking for feedback on whether it would make sense to have misc stats enabled through libcontainer CgroupManager which would then allow me to update cAdvisor or in other places to get misc stats.

Thoughts?

[eero-t]

Somewhat related: #3849

[mythi]

/cc @kolyshkin would be nice to hear initial thoughts on this. I can help with a POC if this makes sense

[mythi]

The code was added to libcontainer.