Update multicluster gateways command to use per cluster metrics

[adleong]

As described in linkerd/rfc#31 we are moving to a model where we will not install the service-mirror-controller as part of linkerd multicluster install but instead create a service mirror controller per linked target cluster as part of linkerd multicluster link. This means that the linkerd multicluster gateways command will need to be updated to account for the fact that there is no longer a singular service mirror controller.

This is hopefully as simple as updating the prometheus scrape config to scrape all service mirror controllers.

Note also that this functionality may eventually be replaced by putting gateway health data directly in the Link resource's Status struct and surfacing it through kubectl get.

[adleong]

Fixed by #4710