docker metrics (read metrics from cgroups for specified container)

Fixes #8842.

This is first and very simple implementation of docker metrics command. A lot of work needed to push it to production.

Roadmap:

* make find of cgroup paths more reliable (not just try to search predefined locations, but get exact value from procfs)
* specify set of metrics (from `cpuacct`, `memory`, `blkio`, ... controllers)
* store metrics in `struct` instead of `map[string]string` (typed data)
* `-f FORMAT` support
* documentation

PS. PR created for further discussion and this branch will be constantly updated.

/cc @shykes @tobegit3hub @thaJeztah
use json.Unmarshal and a struct or map for this, please.