Could you use a password object here. It ensures that we don't accidentally log passwords in plain text. Heres some examples of where its used elsewhere:
https://github.com/logstash/logstash/blob/master/lib/logstash/inputs/rabbitmq.rb#L33
https://github.com/logstash/logstash/blob/master/lib/logstash/inputs/rabbitmq/hot_bunnies.rb#L20