Expose JUJU_AVAILABILITY_ZONE as a node label

[Nobuto Murata]

By default, the charmed deployment will add labels like the following for worker nodes:

$ kubectl get nodes --show-labels
NAME STATUS ROLES AGE VERSION LABELS
calm-alien Ready <none> 3m48s v1.22.4 beta.kubernetes.io/arch=amd64,beta.kubernetes.io/os=linux,juju-application=kubernetes-worker,kubernetes.io/arch=amd64,kubernetes.io/hostname=calm-alien,kubernetes.io/os=linux
...

MAAS and OpenStack in mind as Juju providers set JUJU_AVAILABILITY_ZONE in a charm env with underlying zone/AZ information. It would make sense to expose those information to users as node labels so they can ensure anti-affinity of their workload based on failure domains.

This namespace could be used for that.
https://kubernetes.io/docs/reference/labels-annotations-taints/#topologykubernetesiozone

Here is an example from other charms:
https://jaas.ai/ceph-osd#charm-config-customize-failure-domain

A current workaround is to use a charm option as "labels":
https://jaas.ai/u/containers/kubernetes-worker#charm-config-labels
However, it requires an application in each zone so it's cumbersome from an operational point of view. i.e.,

juju config kubernetes-worker-zone1 labels=topology.kubernetes.io/zone=zone1
juju config kubernetes-worker-zone2 labels=topology.kubernetes.io/zone=zone2
juju config kubernetes-worker-zone3 labels=topology.kubernetes.io/zone=zone3
...

could be something simpler like the following or even with zero option:

juju config kubernetes-worker expose-zone=true