title: watch request to new topic pauses push notifications to existing topic
body: <p>I've configured two topics in my Google Cloud Pub/Sub account, each with a single subscription, under a single project. </p>
  
  <p>When I call watch() on my inbox, passing topic 1 to the watch request, I receive push notifications to my desired endpoint. When I call watch() on my inbox for <em>topic 2</em>, I receive push notifications for the endpoint tied to topic 2, but <strong>push notifications stop arriving for topic 1</strong>. </p>
  
  <p>It appears that only a single topic can push out notifications for a given inbox at any given time, for a given project. Can anyone help confirm whether this is the case?</p>
  
  <p>I can't find anything in <a href="https://developers.google.com/gmail/api/guides/push" rel="nofollow noreferrer">the documentation for Gmail push notifications</a> that notes this limitation. I receive a successful response from the API every time I call watch() again (subscribed to topic 1 -> call watch() for topic 2, and vice versa), but notifications still stop for the topic I was previously subscribed to. I'm also well within the expiry window for the watch() requests.</p>
  
  <p>Any advice / direction would be much appreciated!</p>