Port forward in monitoring should log errors

[slinkydeveloper]

/area test-and-release

Expected Behavior

The code that performs the port forwarding doesn't log any info. We need it in order to diagnose knative/eventing#3570

Actual Behavior

Nothing is logged from kubectl port-forward process started in background

Additional Info

We might think to use directly the client-go to perform port forwarding https://godoc.org/k8s.io/client-go/tools/portforward#New

[slinkydeveloper]

/assign

[slinkydeveloper]

Example on how to perform port forwarding without using kubectl https://github.com/gianarb/kube-port-forward/blob/master/main.go

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