Support the controller to watching a single namespace.

[TingluoHuang]

Part 3 of #2275

• Introduce watchSingleName to gha-runner-scale-set-controller to allow the controller only watch resources in a single namespace.

[nikola-jokic]

I don't know if this would be a good idea, but I'll just ask for discussion:
Should we default to the default namespace if no namespace is provided, like k8s does? It seems more dangerous in case someone makes a mistake, but it also aligns well with the way k8s treats the empty value

[rentziass]

This env var gets its value from

    - name: CONTROLLER_MANAGER_POD_NAMESPACE
      valueFrom:
        fieldRef:
          apiVersion: v1
          fieldPath: metadata.namespace

so it depends on where arc is installed?

[nikola-jokic]

Yes, but I am not sure if it is going to receive "" if no namespace is set ☺️ I'll play around to make sure, because I am not sure how it would look like if metadata.namespace is not set

[rentziass]

It should never be empty though, you can't create a resource outside a namespace?

[nikola-jokic]

Is this a new value that we should set for the part-of?
The ADR says just "actions-runner-controller"
Should we update the ADR?

[rentziass]

Good shout, the ADR pre dates the rename.