Try to avoid reporting stale metrics

[rtyler]

With consumer groups that were short-lived ephemeral consumer groups we should not attempt to report metrics on those up to the metrics provider.

This will prevent us from unnecessarily reporting metrics on consumers which haven't updated anything in, say, two weeks.

[rtyler]

The more I think about this the less convinced I am of one solution over another. Basically Verspaetung registers a ConsumerGauge with the internal MetricsRegistry and the getValue call will simply look up the latest values in the map of KafkaConsumer objects for that topic and TopicPartition objects for the latest metadata.

Since this all happens from within the getValue call we can't very well remove ourselves from the MetricsRegistrythere can we? We would also need to clean up the map with KafkaConsumer objects there too.