e2e: grab controller and scheduler metrics via port forwarding

The previous approach with grabbing via a nginx proxy had some
drawbacks:
- it did not work when the pods only listened on localhost (as
  configured by kubeadm) and the proxy got deployed on a different
  node
- starting the proxy raced with starting the pods, causing
  sporadic test failures because the proxy was not set up
  properly unless it saw all pods when starting the e2e.test
- the proxy was always started, whether it is needed or not
- the proxy was left running after a test and then the next
  test run triggered potentially confusing messages when
  it failed to create objects for the proxy

The new approach is similar to "kubectl port-forward" + "kubectl get
--raw". It uses the port forwarding feature to establish a TCP
connection via a custom dialer, then lets client-go handle TLS and
credentials.

Somehow verifying the server certificate did not work. As this
shouldn't be a big concern for E2E testing, certificate checking gets
disabled on the client side instead of investigating this further.

test/e2e/instrumentation/monitoring/metrics_grabber.go

@@ -51,7 +51,7 @@ var _ = instrumentation.SIGDescribe("MetricsGrabber", func() {
 			}
 		}
 		gomega.Eventually(func() error {
-			grabber, err = e2emetrics.NewMetricsGrabber(c, ec, true, true, true, true, true, true)
+			grabber, err = e2emetrics.NewMetricsGrabber(c, ec, f.ClientConfig(), true, true, true, true, true, true)
 			if err != nil {
 				return fmt.Errorf("failed to create metrics grabber: %v", err)
 			}