middleware/metrics: fix crash on startup (#318)

Make the methods that handle Metrics all use pointer receivers to fix
sync.Once not being initialized.

Finish the setup_test to test for failures. And make the check for the
address more strict and return an error when it does not have a port
number.

Add a toplevel test that starts a CoreDNS server with metrics enabled
so we catch these errors in the future.

test/metrics_test.go

@@ -0,0 +1,17 @@
+package test
+
+import "testing"
+
+// Start test server that has metrics enabled. Then tear it down again.
+func TestMetricsServer(t *testing.T) {
+	corefile := `.:0 {
+	chaos CoreDNS-001 miek@miek.nl
+	prometheus localhost:0
+}
+`
+	srv, err := CoreDNSServer(corefile)
+	if err != nil {
+		t.Fatalf("Could not get CoreDNS serving instance: %s", err)
+	}
+	defer srv.Stop()
+}