derp: specify type of the by-reason drop varz.

Signed-off-by: David Anderson <dave@natulte.net>

derp/derp_server.go

@@ -653,7 +653,7 @@ func (s *Server) ExpVar() expvar.Var {
 	m.Set("bytes_received", &s.bytesRecv)
 	m.Set("bytes_sent", &s.bytesSent)
 	m.Set("packets_dropped", &s.packetsDropped)
-	m.Set("packets_dropped_reason", &s.packetsDroppedReason)
+	m.Set("counter_packets_dropped_reason", &s.packetsDroppedReason)
 	m.Set("packets_sent", &s.packetsSent)
 	m.Set("packets_received", &s.packetsRecv)
 	m.Set("unknown_frames", &s.unknownFrames)