KuebeletStats receiver, fetch metadata from initContainers

[dloucasfx]

Signed-off-by: Dani Louca [email protected]

Description:
kubeletstats is throwing the below error.

2022-07-07T15:19:46.365Z warn kubelet/accumulator.go:112 failed to fetch
container metrics {"kind": "receiver", "name": "kubeletstats", "pod":
"init-app-container", "container": "init-example", "error": "failed to
set extra labels from metadata: pod
\"4406a18b-3d22-4b27-bdae-3c89de5f7868\" with container \"init-example\"
not found in the fetched metadata"

The current implementation does not handle init containers correctly;
The code is only looking for ContainerStatuses but the init containers live under InitContainerStatuses instead

Testing:
Unit test was added and manual validation of the fix has been tetsted using the below pod and making sure the error is no longer in the logs.

apiVersion: v1
kind: Pod
metadata:
  name: init-app-container
  labels:
    app: initapp
spec:
  containers:
  - name: myapp-container
    image: busybox:1.28
    command: ['sh', '-c', 'sleep 360000']
  initContainers:
  - name: init-example
    image: busybox:1.28
    command: ['sh', '-c', "sleep 20"]

Documentation: