Add logging for succesful broadcast message creation

app/v2/broadcast/post_broadcast.py

@@ -1,5 +1,5 @@
 from itertools import chain
-from flask import jsonify, request
+from flask import current_app, jsonify, request
 from notifications_utils.polygons import Polygons
 from app import authenticated_service, api_user
 from app.broadcast_message.translators import cap_xml_to_dict
@@ -63,4 +63,9 @@ def create_broadcast():
 
     dao_save_object(broadcast_message)
 
+    current_app.logger.info(
+        f'Broadcast message {broadcast_message.id} created for service '
+        f'{authenticated_service.id} with reference {broadcast_json["reference"]}'
+    )
+
     return jsonify(broadcast_message.serialize()), 201