Call start when creating a background thread transport (#3412)

Author: Jon Wayne Parrott

packages/google-cloud-logging/google/cloud/logging/handlers/transports/background_thread.py

@@ -247,6 +247,7 @@ def __init__(self, client, name, grace_period=_DEFAULT_GRACE_PERIOD,
             client.project, client._credentials, http)
         logger = self.client.logger(name)
         self.worker = _Worker(logger)
+        self.worker.start()
 
     def send(self, record, message):
         """Overrides Transport.send().