Update stopwatch to include error handling with unit tests (#510)

Co-authored-by: Daniel McKnight <daniel@neon.ai>

neon_utils/metrics_utils.py

@@ -67,7 +67,11 @@ def start(self):
         self.start_time = time()
 
     def stop(self):
-        self.time = time() - self.start_time
+        try:
+            self.time = time() - self.start_time
+        except TypeError:
+            LOG.error("stop called before start!")
+            self.time = None
         return self.time
 
     def report(self):

tests/metric_util_tests.py

@@ -53,6 +53,10 @@ def test_stopwatch_simple(self):
             sleep(sleep_time)
         self.assertEqual(round(stopwatch.time, 2), sleep_time)
 
+        stopwatch = Stopwatch()
+        stopwatch.stop()
+        self.assertIsNone(stopwatch.time)
+
     def test_stopwatch_reuse(self):
         sleep_time = 0.5
         stopwatch = Stopwatch()