Per-CPU stats are more expensive to transport and store, and that
level of detail is not required in many cases.
We export overall total cpu in the same metric as per-cpu, so that
dashboards which previously summed over cpu will work identically.
As of the 4.7 kernel, the cpustats field returned from libcontainer
contains values for every possible cpu (including nonexistent ones).
The extra values are all 0s.
If we assume that hotplug events won't happen, we can get a more
accurage cpu count by using runtime.NumCPU and then ignoring any values
beyond that.
