initContainer metrics fail

[kfox1111]

What happened:
the metrics-server is unable to get metrics for pods with init containers.
See also: kubernetes-sigs/metrics-server#213 (comment)

What you expected to happen:
Metrics for pods with init containers show up.

How to reproduce it (as minimally and precisely as possible):
create a pod with an init container. metrics are not gathered for that pod.

Environment:

• Kubernetes version (use kubectl version):
  1.13.2
• Runtime:
  containerd 1.2.2

[kfox1111]

@kubernetes/sig-node-bugs

[k8s-ci-robot]

@kfox1111: Reiterating the mentions to trigger a notification:
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