Support cgroup v2

[MatMaul]

For now RSS, open FDs and sockets are not supported.

[MatMaul]

There is also a second commit adding total cpu usage (vs only user).
I didn't push an independent PR because it would conflict, but I can remove it and send it later (after the first is merged) if you want.
Since it is pretty small, simple and in an independent commit I thought it was ok to bundle it.

[MatMaul]

Oh I forgot it also add mem+swap metric hum. let me know if you want to break it further completely or if this is fine for review.

[atotto]

Thank you for your PR! I will review this commit.

[MatMaul]

Thanks for the review ! I merged your suggestions.

[atotto]

Did you get the container_memsw_usage_bytes metrics in cgroup-v1 (Legacy and Hybrid) system?

I cannot read it (all container_memsw_usage_bytes are zero)

container_memsw_usage_bytes{id="/system.slice/ssh.service"} 0
container_memsw_usage_bytes{id="/system.slice/avahi-daemon.service"} 0
container_memsw_usage_bytes{id="/system.slice/NetworkManager.service"} 0

We need test environment..