eventlogging-reporter: use metric type and aggregate

* In addition to each named raw / valid stream pair, have an 'overall' stream
  pair which represents the total rate of events going through the system.
* Use a metric type ('|m') rather than counter, so that StatsD calculates
  rate-of-change.
* Make the metric prefix specifiable via the command-line.

Change-Id: I673aa61ab5d700fec5b6f089d8f24be59d6a3c89