PubSub usage must be monitored

The PubSub usage with Nuxeo Stream implementation is not monitored.

The implementation is using low-level producer-consumer API where the consumer doesn't commit its position because it doesn't need to keep up with the old messages.

Kafka is not able to list consumers that don't commit their position, as a result, the pubsub consumers are not visible by the monitoring.

Still, invalidations (repository, directory caches) are using pubsub and can create lots of activities that need to be reported.

A simple pubsub counter on pub should be enough to have an idea.