added dht_invalid_find_node counter metric

include/libtorrent/performance_counters.hpp

@@ -238,6 +238,7 @@ namespace libtorrent
 
 			dht_invalid_announce,
 			dht_invalid_get_peers,
+			dht_invalid_find_node,
 			dht_invalid_put,
 			dht_invalid_get,
 

src/kademlia/node.cpp

@@ -863,6 +863,7 @@ void node::incoming_request(msg const& m, entry& e)
 		bdecode_node msg_keys[2];
 		if (!verify_message(arg_ent, msg_desc, msg_keys, error_string))
 		{
+			m_counters.inc_stats_counter(counters::dht_invalid_find_node);
 			incoming_error(e, error_string);
 			return;
 		}

src/session_stats.cpp

@@ -454,6 +454,7 @@ namespace libtorrent
 		// the number of failed incoming DHT requests by kind of request
 		METRIC(dht, dht_invalid_announce)
 		METRIC(dht, dht_invalid_get_peers)
+		METRIC(dht, dht_invalid_find_node)
 		METRIC(dht, dht_invalid_put)
 		METRIC(dht, dht_invalid_get)