stream.sh monitor or traker commands don't refresh consumer groups

When running stream.sh to monitor the lag and latency (or track them into another stream), the list of consumers for a stream is computed at launch time.

The command will not report any metrics if it is run before consumer groups have been started (Nuxeo instances are fully initialized).

The list of consumer groups in a Kafka cluster is dynamic and should be periodically re-evaluated in order to work even when the monitoring is started while the infra is still initializing.