Resolves prometheus#77 add cmdline flag to drop unmapped

* Adds `-statsd.drop-unmapped` cmdline flag which causes statsd metrics
  that don't match a mapping to be dropped. New flag defaults to "false"

exporter.go

@@ -228,6 +228,9 @@ func (b *Exporter) Listen(e <-chan Events) {
 				}
 			} else {
 				eventsUnmapped.Inc()
+				if *dropUnmapped == true {
+					continue
+				}
 				metricName = escapeMetricName(event.MetricName())
 			}
 

main.go

@@ -39,6 +39,7 @@ var (
 	readBuffer          = flag.Int("statsd.read-buffer", 0, "Size (in bytes) of the operating system's transmit read buffer associated with the UDP connection. Please make sure the kernel parameters net.core.rmem_max is set to a value greater than the value specified.")
 	addSuffix           = flag.Bool("statsd.add-suffix", true, "Add the metric type (counter/gauge/timer) as suffix to the generated Prometheus metric (NOT recommended, but set by default for backward compatibility).")
 	showVersion         = flag.Bool("version", false, "Print version information.")
+	dropUnmapped        = flag.Bool("statsd.drop-unmapped", false, "Drop statsd metrics which have no matched mapping configured.")
 )
 
 func serveHTTP() {