Fixes an issue where flow_stats reply in ofagent fails.

When there is no metadata, i.e. it is zero, the write-metadata field
in the of flow_stats_reply should adjust accordingly. Also added some
debug logs for packet-in/out on both sides of grpc channel between
ofagent and core.

Change-Id: I8ad2955ec707947b4c7e0af138bcc0238cf3c4c2
diff --git a/ofagent/grpc_client.py b/ofagent/grpc_client.py
index 018baa9..6e9765f 100644
--- a/ofagent/grpc_client.py
+++ b/ofagent/grpc_client.py
@@ -151,11 +151,13 @@
             packet_in = yield self.packet_in_queue.get()
             device_id = packet_in.id
             ofp_packet_in = packet_in.packet_in
+            self.log.debug('grpc client to send packet-in')
             self.connection_manager.forward_packet_in(device_id, ofp_packet_in)
             if self.stopped:
                 break
 
     def send_packet_out(self, device_id, packet_out):
+        self.log.debug('grpc client to send packet-out')
         packet_out = PacketOut(id=device_id, packet_out=packet_out)
         self.packet_out_queue.put(packet_out)