title: Monitoring Pub/Sub - Filter by attributes
body: <p>I want to monitor Pub/Sub topic messages by filtering a certain attribute or message content. In detail, the pubsub message is in json format, following a schema. There is one field in the message indicating the publisher client source. For the same topic, I have multiple publisher clients to publish to it, so I want to set up alert that will notify me if one client stops publishing messages to this topic.</p>
  <p>Since if one client fails to publish, there are other clients still publishing to the topic, so total count doesn't help, I need filter by the client source field in the message. Is there anyway to do this in alert please? Appreciate any suggestion!</p>