title: Firebase PubSub Emulator not recieving messages
body: <p>I'm trying to get the firebase emulator, to trigger cloud functions using pubsub publishing.
  The basic setup works outside of the cloud functions.
  When trying to connect the cloud function, no message ever appears.</p>
  <p>I have the feeling, that I doing something fundamentally wrong.</p>
  <p>Publish code:</p>
  <pre><code>import { PubSub, Topic } from '@google-cloud/pubsub'; 
  const topicName = 'MyTopic';
  const [taskTopic] = await pubsub.createTopic(topicName);
  await this.taskTopic.publish(Buffer.from(msg));
  
  </code></pre>
  <p>&quot;Manual&quot; Subscription: works</p>
  <pre><code> [subscription] = await this.renderTaskTopic.createSubscription(subscriptionName);
   subscription.on('message', (message) =&gt; {
      console.log('Received message:', message.data.toString());
      process.exit(0);
   });
  
  </code></pre>
  <p>Subscription using cloud function: never gets called in the emulator</p>
  <pre><code>
  export const subscribeToRenderTask = functions.pubsub.topic(topicName)
      .onPublish(async (message: functions.pubsub.Message) =&gt; {
          console.log('subscribeToRenderTask', message.data);
       }
  </code></pre>
  <p>The emalutor setup seems fine</p>
  <pre><code> functions: The Firebase Authentication emulator is not running, so calls to Firebase Authentication will affect production.
  ✔  functions[convert]: http function initialized (http://0.0.0.0:5001/puredio-development/europe-west3/convert).
  ✔  functions[subscribeToRenderTask]: pubsub function initialized.
  
  ┌─────────────────────────────────────────────────────────────┐
  │ ✔  All emulators ready! It is now safe to connect your app. │
  │ i  View Emulator UI at http://0.0.0.0:8081                  │
  └─────────────────────────────────────────────────────────────┘
  
  ┌───────────┬────────────────┬───────────────────────────────┐
  │ Emulator  │ Host:Port      │ View in Emulator UI           │
  ├───────────┼────────────────┼───────────────────────────────┤
  │ Functions │ 0.0.0.0:5001   │ http://0.0.0.0:8081/functions │
  ├───────────┼────────────────┼───────────────────────────────┤
  │ Firestore │ 0.0.0.0:8080   │ http://0.0.0.0:8081/firestore │
  ├───────────┼────────────────┼───────────────────────────────┤
  │ Pub/Sub   │ 127.0.0.1:8085 │ n/a                           │
  └───────────┴────────────────┴───────────────────────────────┘
    Emulator Hub running at 127.0.0.1:4400
    Other reserved ports: 4500
  </code></pre>