Fix noop metrics timed decorator

Previously if you did not have the monasca statds available, the @timed
decorator would result in a NoneType error because timed was returning
None instead of acting as a noop function wrapper. This change updates
the NoopTimer to properly handle timed being used as a function.

Change-Id: Ic5543b3503b24aaef960d8b7c0cb38940241cbcb
Closes-Bug: #1659638

designate/metrics_client/noop.py

@@ -69,7 +69,9 @@ class NoopTimer(object):
         pass
 
     def timed(self, *a, **kw):
-        pass
+        def wrapper(func):
+            return func
+        return wrapper
 
 
 class Client(object):

designate/tests/unit/test_metrics/test_metrics.py

@@ -53,6 +53,15 @@ class TestNoopMetrics(TestCase):
         self.assertIsInstance(self.metrics.timer(), noop.NoopTimer)
         self.assertIsNotNone(self.metrics.timed.__self__)
 
+    def test_noop_metrics_client_timed(self):
+        timer = self.metrics._client.get_timer()
+
+        @timer.timed('timed.test')
+        def func(a):
+            return a
+        result = func(1)
+        self.assertEqual(result, 1)
+
 
 class TestMonascaMetrics(TestCase):
 
@@ -86,3 +95,12 @@ class TestMonascaMetrics(TestCase):
         self.assertIsInstance(self.metrics.timer(),
                               monascastatsd.timer.Timer)
         self.assertIsNotNone(self.metrics.timed.__self__)
+
+    def test_monasca_metrics_client_timed(self):
+        timer = self.metrics._client.get_timer()
+
+        @timer.timed('timed.test')
+        def func(a):
+            return a
+        result = func(1)
+        self.assertEqual(result, 1)