Print Cgroup stats. Useful to find out if a test is getting CPU throttled.
Print Cgroup stats. Useful to find out if a test is getting CPU throttled.

Summary:
Appears in log file like:
```
2015/10/21 01:18:05 [lxc] Cgroup stats: [nr_periods 189 nr_throttled 47 throttled_time 4185982625]
```

- nr_periods is the number of times CPU limit was checked (since period is 100ms, this is usually
   total wall clock time * 10)
- nr_throttled is number of periods where throttling was applied (this should ideally be 0, or a
   very small fraction of nr_periods). Large percentage of nr_period (> ~25%) means heavy throttling.
   We should consider alerting on this.
- throttled_time is amount of time in nanoseconds which the process could have used had throttling
   not been in place. Don't read much into this.

Test Plan: Unit tests passed

Reviewers: kylec

Reviewed By: kylec

Subscribers: changesbot, mkedia, nate

Differential Revision: https://tails.corp.dropbox.com/D147274

dropbox/changes-client,dropbox/changes-client