With the Kafka $source customers can track the position of the stream processor in the topic to understand how far behind it is. Today there is no way to monitor how far behind a change stream $source is in the oplog from being current. We should develop a way to track this lag for customers to monitor and alert on. 

The Kafka Connector has the ability to do something like this today with a jmx metrics called 

https://www.mongodb.com/docs/kafka-connector/current/monitoring/#source-connector-jmx-metrics