Reset counter (#1005)

* Add .reset method to Counter metric
* Update method docstring
* Add a test for .reset() method

Signed-off-by: Paul Melnikov <positron96@gmail.com>
Co-authored-by: Chris Marchbanks <csmarchbanks@gmail.com>

Author: positron96

prometheus_client/metrics.py

@@ -292,6 +292,12 @@ def f():
         # Count only one type of exception
         with c.count_exceptions(ValueError):
             pass
+            
+    You can also reset the counter to zero in case your logical "process" restarts
+    without restarting the actual python process.
+
+       c.reset()
+
     """
     _type = 'counter'
 
@@ -310,6 +316,11 @@ def inc(self, amount: float = 1, exemplar: Optional[Dict[str, str]] = None) -> N
             _validate_exemplar(exemplar)
             self._value.set_exemplar(Exemplar(exemplar, amount, time.time()))
 
+    def reset(self) -> None:
+        """Reset the counter to zero. Use this when a logical process restarts without restarting the actual python process."""
+        self._value.set(0)
+        self._created = time.time()
+
     def count_exceptions(self, exception: Union[Type[BaseException], Tuple[Type[BaseException], ...]] = Exception) -> ExceptionCounter:
         """Count exceptions in a block of code or function.
 

tests/test_core.py

@@ -43,6 +43,16 @@ def test_increment(self):
         self.counter.inc(7)
         self.assertEqual(8, self.registry.get_sample_value('c_total'))
 
+    def test_reset(self):
+        self.counter.inc()
+        self.assertNotEqual(0, self.registry.get_sample_value('c_total'))
+        created = self.registry.get_sample_value('c_created')
+        time.sleep(0.05)
+        self.counter.reset()        
+        self.assertEqual(0, self.registry.get_sample_value('c_total'))
+        created_after_reset = self.registry.get_sample_value('c_created')
+        self.assertLess(created, created_after_reset)       
+
     def test_repr(self):
         self.assertEqual(repr(self.counter), "prometheus_client.metrics.Counter(c)")