add a stats counter for clients dropped due to recursive-clients limit

(cherry picked from commit 715afa9c57)

lib/ns/client.c

@@ -278,8 +278,11 @@ ns_client_killoldestquery(ns_client_t *client) {
 		ISC_LIST_UNLINK(client->manager->recursing, oldest, rlink);
 		UNLOCK(&client->manager->reclock);
 		ns_query_cancel(oldest);
-	} else
+		ns_stats_increment(client->sctx->nsstats,
+				   ns_statscounter_reclimitdropped);
+	} else {
 		UNLOCK(&client->manager->reclock);
+	}
 }
 
 void

lib/ns/include/ns/stats.h

@@ -104,7 +104,9 @@ enum {
 
 	ns_statscounter_tcphighwater = 65,
 
-	ns_statscounter_max = 66,
+	ns_statscounter_reclimitdropped = 66,
+
+	ns_statscounter_max = 67,
 };
 
 void