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.

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!