Is your feature request related to a problem? Please describe.
Hi,

I have a simple publish for loop:

for message in data:
        future = publisher.publish(topic_path, data=message)
        future.add_done_callback(self.get_callback(future, message))

with some callback (-> I don't wait until the future is returned). The problem is that when the last batch of data is being created, the process terminates earlier than it's being sent (<MAX_LATENCY), missing some last messages.

Could there be a way of something like publisher.join() or publisher.force_push() to make sure that all messages are sent before the process terminates?

Related:

• Pub / Sub Publisher: Async publish breaks around 1000 messages #4575
• PubSub Publisher: sometime cannot publish until a number of publish called #4607
• https://github.com/googleapis/google-cloud-python/issues/4994