retainAckedMessages

Indicates whether to retain acknowledged messages. If true, then messages are not expunged from the subscription's backlog, even if they are acknowledged, until they fall out of the message_retention_duration window. This must be true if you would like to `Seek` to a timestamp (https://cloud.google.com/pubsub/docs/replay-overview#seek_to_a_time) in the past to replay previously-acknowledged messages.