Health tracker for a Pub/Sub subscription. The tracker will monitor messages being processed and
the subscription's message backlog. If backlog message size exceeds the message backlog
threshold, the tracker will return the number of messages over the threshold.
Fields inherited from interface com.google.cloud.spring.pubsub.core.health.HealthTracker
If messages have been processed recently returns zero. If no message have been recently
processed the tracker will retrieve the number of messages in the subscription backlog. If the
backlogs exceeds the threshold the messages over the threshold shall be returned.