[aws-cloudwatch-metrics] Support priorityClassName

Please add:

To [daemonset.yaml](https://github.com/aws/eks-charts/blob/master/stable/aws-cloudwatch-metrics/templates/daemonset.yaml):
# ...
spec:
{{- if .Values.priorityClassName }}
priorityClassName: "{{ .Values.priorityClassName }}"
{{- end }}
containers:
# ...
To values.yaml:
# ...

# Leverage a PriorityClass to ensure your pods survive resource shortages
# ref: https://kubernetes.io/docs/concepts/configuration/pod-priority-preemption/
priorityClassName: "" # system-cluster-critical
...

Start by reading stable/aws-cloudwatch-metrics/templates/daemonset.yaml and the chart's values.yaml. Confirm that the optional priorityClassName value is represented in values.yaml and is emitted in the DaemonSet pod spec when set, while remaining absent when empty.

Written by the indexing model from the issue text.