Add support for active/inactive for a revision. about serving

[mattmoor]

Coordinate with @josephburnett and #autoscaling

[drewinglis]

@josephburnett and I had a long conversation about this today. Here's the new proposed spec:

Spec.Addressable will be set by the RouteController. It will be true if there are any traffic splits in the current set of Route.Spec or Route.Status traffic splits that include this Revision, and false otherwise. If it is false, we should delete all Kuberenetes resources and Istio routes used by this revision.
If true, we should create all Kubernetes resources and Istio routes (routing dependent on the state of Spec.Active below).

Spec.Active will be set by the autoscaler/activator. It will be true if the Revision is serving or has recently served traffic, and false if it has not received traffic recently. If it's true, the Istio routes should point to the pods, and if it's false, the Istio routes should point to the activator.

[mattmoor]

Multiple bools sounds like why K8s-style prefers enums over bools. Can we make that make sense?

[mattmoor]

@drewinglis @josephburnett We now have RevisionServingStateType, so what work is left here?

[drewinglis]

I'm not sure what the scope of this issue is exactly. If it was only to add the label in the API, then we can close it. If it was also tracking the work to set it, then it should stay open.

[josephburnett]

The scope includes the implementation. Updated description to include that.

[mattmoor]

I believe this is checked in as part of an autoscaler experiment, and done barring broader model changes in M5 and beyond, which are tracked separately. I'm going to close this out.

If I'm wrong, please re-open :)