Add telemetry to detect stuck rolling updates

What does this PR do?

Add a new metric ers_rolling_update_stuck: 1 if the number of unavailable pods is higher than maxUnavailable, 0 otherwise.

Motivation

To be alerted if an ERS rolling update get stuck because of the number of unavailable pods in the cluster.

Describe your test plan

Deploy the EDS

Apply a CR with a low maxUnavailable value (min supported value is 1)

apiVersion: datadoghq.com/v1alpha1
kind: ExtendedDaemonSet
metadata:
name: pause-containers
spec:
strategy:
rollingUpdate:
  maxUnavailable: 1
template:
spec:
  containers:
  - name: daemon
    image: k8s.gcr.io/pause:3.0

After the deployment is done, trigger a rolling update that will cause the new pods to be available,. To do so, a broken readiness probe can be added. This will make the EDS consider the new pod unavailable and should throttle/stuck the rolling update

apiVersion: datadoghq.com/v1alpha1
kind: ExtendedDaemonSet
metadata:
name: pause-containers
spec:
strategy:
rollingUpdate:
  maxUnavailable: 1
template:
spec:
  containers:
  - name: daemon
    image: k8s.gcr.io/pause:3.0
    readinessProbe:
      tcpSocket:
        port: 8080

Make sure the metric shows that the new ers is stuck

$ curl localhost:8080/metrics | grep ers_rolling_update_stuck
# HELP ers_rolling_update_stuck 1 if the number of unavailable pods is higher than maxUnavailable, 0 otherwise
# TYPE ers_rolling_update_stuck gauge
ers_rolling_update_stuck{name="pause-containers-67zbv",namespace="default"} 0
ers_rolling_update_stuck{name="pause-containers-vpkxk",namespace="default"} 1

DataDog / extendeddaemonset