Adding log message when Pub/Sub maintain_leases exits. (#4484)

Author: dhermes

google/cloud/pubsub_v1/subscriber/policy/base.py

@@ -309,6 +309,7 @@ def maintain_leases(self):
         while True:
             # Sanity check: Should this infinite loop quit?
             if not self._consumer.active:
+                _LOGGER.debug('Consumer inactive, ending lease maintenance.')
                 return
 
             # Determine the appropriate duration for the lease. This is