dht: Add counters for writes and write errors

dht/expvar.go

@@ -11,4 +11,6 @@ var (
 	readUnmarshalError = expvar.NewInt("dhtReadUnmarshalError")
 	readQuery          = expvar.NewInt("dhtReadQuery")
 	announceErrors     = expvar.NewInt("dhtAnnounceErrors")
+	writeErrors        = expvar.NewInt("dhtWriteErrors")
+	writes             = expvar.NewInt("dhtWrites")
 )

dht/server.go

@@ -382,7 +382,9 @@ func (s *Server) writeToNode(b []byte, node Addr) (err error) {
 		}
 	}
 	n, err := s.socket.WriteTo(b, node.UDPAddr())
+	writes.Add(1)
 	if err != nil {
+		writeErrors.Add(1)
 		err = fmt.Errorf("error writing %d bytes to %s: %s", len(b), node, err)
 		return
 	}