Add debugging info for GCP enrich issue

We try to find out where the UUID object that cannot be serialized comes
from.

zentral/core/queues/backends/google_pubsub/consumer.py

@@ -177,7 +177,12 @@ def producer_client(self):
         return pubsub_v1.PublisherClient(credentials=self.credentials)
 
     def publish_event(self, event, machine_metadata):
-        message = json.dumps(event.serialize(machine_metadata=machine_metadata)).encode("utf-8")
+        serialized_event = event.serialize(machine_metadata=machine_metadata)
+        try:
+            message = json.dumps(serialized_event).encode("utf-8")
+        except TypeError:
+            logger.error("Could not serialize event %s", serialized_event)
+            raise
         kwargs = {"event_type": event.event_type}
         if event.metadata.routing_key:
             kwargs["routing_key"] = event.metadata.routing_key