Serving: Activator leaks metrics publicly

[scothis]

Expected Behavior

When making a request to a deactive service /metrics, I expect the service to activate and handle the request.

Actual Behavior

The activator's /metrics endpoint hijacks the request and returns it's own internal metrics

Steps to Reproduce the Problem

1. create a service
2. wait for it to scale to zero
3. make a request to the service with the path /metrics
4. see that the response is activator's metrics

Additional Info

Metrics and other control plane request handlers must run on a different port in order to not collide with the data plane.

[tcnghia]

/cc @akyyy @josephburnett

[josephburnett]

I think we should move the metrics exporting to another port.

[akyyy]

/cc @mdemirhan