I'm attempting to ingest data from PubSub via Apache Spark. Without getting into details, I would like to acknowledge messages outside of the MessageReceiver interface. I am not seeing any public methods that acknowledge messages outside of the MessageReceiver interface.
https://cloud.google.com/pubsub/docs/pull

I see that a REST endpoint exists here:
https://cloud.google.com/pubsub/docs/reference/rest/v1/projects.subscriptions/acknowledge

But I am unable to find an accompanying method in the legacy api:
https://developers.google.com/resources/api-libraries/documentation/pubsub/v1/java/latest/

Is there any way to manually acknowledge messages in this (google-cloud) or the the legacy APIs?