Expose measured duration on Timer context manager (prometheus#1166)

Assigning the .time() context manager (with ... as t) now yields a Timer
whose .duration attribute holds the observed value in seconds after the
block exits. This lets callers reuse the measurement (logging, further
metrics) without calling default_timer() a second time.

Signed-off-by: Lukáš Vokráčko <lukas@vokracko.cz>

Author: vokracko

docs/content/instrumenting/gauge.md

@@ -108,6 +108,10 @@ def process():
 
 with g.time():
     pass
+
+with g.time() as t:
+    pass
+print(t.duration) # observed time in seconds.
 ```
 
 ### `set_function(f)`

docs/content/instrumenting/histogram.md

@@ -86,6 +86,10 @@ def process():
 
 with h.time():
     pass
+
+with h.time() as t:
+    pass
+print(t.duration) # observed time in seconds.
 ```
 
 ## Labels

docs/content/instrumenting/summary.md

@@ -68,6 +68,10 @@ def process():
 
 with s.time():
     pass
+
+with s.time() as t:
+    pass
+print(t.duration) # observed time in seconds.
 ```
 
 ## Labels

prometheus_client/context_managers.py

@@ -55,6 +55,7 @@ class Timer:
     def __init__(self, metric, callback_name):
         self._metric = metric
         self._callback_name = callback_name
+        self.duration = None
 
     def _new_timer(self):
         return self.__class__(self._metric, self._callback_name)
@@ -65,9 +66,9 @@ def __enter__(self):
 
     def __exit__(self, typ, value, traceback):
         # Time can go backwards.
-        duration = max(default_timer() - self._start, 0)
+        self.duration = max(default_timer() - self._start, 0)
         callback = getattr(self._metric, self._callback_name)
-        callback(duration)
+        callback(self.duration)
 
     def labels(self, *args, **kw):
         self._metric = self._metric.labels(*args, **kw)

tests/test_core.py

@@ -378,6 +378,14 @@ def test_block_decorator_with_label(self):
             metric.labels('foo')
         self.assertEqual(1, value('s_with_labels_count', {'label1': 'foo'}))
 
+    def test_timer_duration_exposed(self):
+        with self.summary.time() as t:
+            time.sleep(0.01)
+        self.assertIsNotNone(t.duration)
+        self.assertGreater(t.duration, 0)
+        recorded_sum = self.registry.get_sample_value('s_sum')
+        self.assertEqual(t.duration, recorded_sum)
+
     def test_timer_not_observable(self):
         s = Summary('test', 'help', labelnames=('label',), registry=self.registry)