Add Container Metrics Support

[Random-Liu]

According to CRI, container runtime is responsible to provide container metrics, including:

• container CPU usage.
• container memory usage.
• container writable layer size.
• Image FS usage.

See here

Containerd should have provided all those information:

• container CPU/memory usage: Provided through Prometheus metrics endpoint.
• container writable layer size: Provided by snapshotter.
• Image FS usage:
  • May need to count size of each snapshot;
  • May need to consider content store.

We should support these container metrics.

/cc @kubernetes-incubator/maintainers-cri-containerd