TCP & UDP Connections By Cgroup

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I haven't found a great way to show the number of TCP / UDP connections by processes in a cgroup. I propose the following:

1. Iterate through PIDs in a cgroup
2. Look at /proc//net/tcp for the number of TCP connections for a given process.
3. Sum up the number of connections.

github.com/prometheus/procfs should be able to help with the heavy lifting of fetching TCP metrics.

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Implemented basic socket stats by cgroup in bdd970a. Still need to:
1.) Refactor this to include these stats for cgroups v1.
2.) Include more stats in verbose mode.