LoggingMetricsConsumer, to dump metrics to log file

* Added a LoggingMetricsConsumer example of a MetricsConsumer. Use it by adding 'conf.registerMetricsConsumer(LoggingMetricsConsumer.class);' when you construct your topology.
* Added a dedicated appender to the example logback/cluster.xml called METRICS.
  Its log format omits the class and priority since with the dedicated appender those are redu