Activator should be healthy iff it is in contact with stat server

[mattmoor]

In what area(s)?

/area autoscale
/area networking

What version of Knative?

HEAD

Expected Behavior

The activator fundamentally needs to be able to send metrics to the autoscaler to perform its function, if that connection hasn't been established, or gets broken, then the activator must start failing health checks.

Actual Behavior

The activator happily accepts requests and simply logs that it hasn't connected to the stats endpoint yet.

Steps to Reproduce the Problem

If you disable the stats endpoint on the autoscaler (scraping will still work above zero) then things will fail to scale from zero despite the activator looking perfectly healthy.

[mattmoor]

We should add a handler here that exposes a way to health check things and back it with status from the sink here

[markusthoemmes]

Related to #2857

[mattmoor]

I see a PR from @jsanda that's somewhat inactive, but doesn't cover this. Do you know if he's actively working on this?

[mattmoor]

I need something for idle hands on the shuttle, I'll see how far I get :)