Also allow to add deferred labels for 'gauge' and 'summary' metrics

For this to work the 'observability' check needs to be deferred as
well, in case a label is added inside the context manager thereby
making the metric observable.

prometheus_client/context_managers.py

@@ -55,6 +55,8 @@ def __enter__(self):
     def __exit__(self, typ, value, traceback):
         # Time can go backwards.
         duration = max(default_timer() - self._start, 0)
+        instance = self._callback.__self__
+        instance._raise_if_not_observable()
         self._callback(duration)
 
     def labels(self, *args, **kw):

prometheus_client/metrics.py

@@ -396,7 +396,6 @@ def time(self):
 
         Can be used as a function decorator or context manager.
         """
-        self._raise_if_not_observable()
         return Timer(self.set)
 
     def set_function(self, f):
@@ -475,7 +474,6 @@ def time(self):
 
         Can be used as a function decorator or context manager.
         """
-        self._raise_if_not_observable()
         return Timer(self.observe)
 
     def _child_samples(self):

tests/test_core.py

@@ -222,13 +222,25 @@ def test_time_block_decorator(self):
             time.sleep(.001)
         self.assertNotEqual(0, self.registry.get_sample_value('g'))
 
+    def test_time_block_decorator_with_label(self):
+        value = self.registry.get_sample_value
+        self.assertEqual(None, value('g2', {'label1': 'foo'}))
+        with self.gauge_with_label.time() as metric:
+            metric.labels('foo')
+        self.assertLess(0, value('g2', {'label1': 'foo'}))
+
     def test_track_in_progress_not_observable(self):
         g = Gauge('test', 'help', labelnames=('label',), registry=self.registry)
         assert_not_observable(g.track_inprogress)
 
     def test_timer_not_observable(self):
         g = Gauge('test', 'help', labelnames=('label',), registry=self.registry)
-        assert_not_observable(g.time)
+
+        def manager():
+            with g.time():
+                pass
+
+        assert_not_observable(manager)
 
 
 class TestSummary(unittest.TestCase):
@@ -318,10 +330,21 @@ def test_block_decorator(self):
             pass
         self.assertEqual(1, self.registry.get_sample_value('s_count'))
 
+    def test_block_decorator_with_label(self):
+        value = self.registry.get_sample_value
+        self.assertEqual(None, value('s_with_labels_count', {'label1': 'foo'}))
+        with self.summary_with_labels.time() as metric:
+            metric.labels('foo')
+        self.assertEqual(1, value('s_with_labels_count', {'label1': 'foo'}))
+
     def test_timer_not_observable(self):
         s = Summary('test', 'help', labelnames=('label',), registry=self.registry)
 
-        assert_not_observable(s.time)
+        def manager():
+            with s.time():
+                pass
+
+        assert_not_observable(manager)
 
 
 class TestHistogram(unittest.TestCase):