Dropwizard Metrics is a very prevalent and easy to use framework for collecting metrics for some application. It contains a number of reporting mechanisms (e.g. console, Graphite, Ganglia) and provides an interface to create custom reporting mechanisms.
The Reporter referred to here is an implementation that supports sending the metrics collected in Dropwizard Metrics and send them to the configured Apache Hadoop Metrics2 system. This allows applications to use the Dropwizard Metrics API but send the metrics information to Metrics2 sinks (e.g. Ambari Metrics System).