https://github.com/GoogleCloudPlatform/google-cloud-java/blob/master/google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/spi/v1/Publisher.java#L183 `addCallback` is no longer available on `ApiFuture`, it is now a static method in `ApiFutures`.
https://github.com/GoogleCloudPlatform/google-cloud-java/blob/master/google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/spi/v1/Publisher.java#L183
addCallbackis no longer available onApiFuture, it is now a static method inApiFutures.