stats: consider ways to extract RSS value from cgroup data

> when Docker changed their API because of the memory values available cgroups v2 we got a lot of issues asking about how to get RSS now that it's not exposed. We might want to considering figuring out how to extract RSS from this value (which I think covers RSS + ...?)

https://github.com/hashicorp/nomad-driver-exec2/pull/1#discussion_r1543625323