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.

middleware/metrics/setup_test.go

@@ -10,19 +10,32 @@ func TestPrometheus(t *testing.T) {
 	tests := []struct {
 		input     string
 		shouldErr bool
+		addr      string
 	}{
-		{`prometheus`, false},
-		{`prometheus {}`, false},   // TODO(miek): should be true
-		{`prometheus /foo`, false}, // TODO(miek): should be true
-		{`prometheus localhost:53`, false},
+		// oks
+		{`prometheus`, false, "localhost:9153"},
+		{`prometheus localhost:53`, false, "localhost:53"},
+		// fails
+		{`prometheus {}`, true, ""},
+		{`prometheus /foo`, true, ""},
 	}
 	for i, test := range tests {
 		c := caddy.NewTestController("dns", test.input)
-		err := setup(c)
+		m, err := prometheusParse(c)
 		if test.shouldErr && err == nil {
 			t.Errorf("Test %v: Expected error but found nil", i)
+			continue
 		} else if !test.shouldErr && err != nil {
 			t.Errorf("Test %v: Expected no error but found error: %v", i, err)
+			continue
+		}
+
+		if test.shouldErr {
+			continue
+		}
+
+		if test.addr != m.Addr {
+			t.Errorf("Test %v: Expected address %s but found: %s", i, test.addr, m.Addr)
 		}
 	}
 }