The ruby lib has a very useful feature, that I'd like to see on the golang lib. A received message can modify their own ack_deadline: https://github.com/GoogleCloudPlatform/google-cloud-ruby/blob/master/google-cloud-pubsub/lib/google/cloud/pubsub/received_message.rb#L159

The go version only allow a call for Ack/Nack: https://github.com/GoogleCloudPlatform/google-cloud-go/blob/master/pubsub/message.go

Does it make sense? This is something I'm willing to send a PR if you agree.