Only mark unavailable server as traversed

Author: chenBright

src/brpc/policy/weighted_randomized_load_balancer.cpp

@@ -139,8 +139,8 @@ int WeightedRandomizedLoadBalancer::SelectServer(const SelectIn& in, SelectOut*
             std::lower_bound(s->server_list.begin(), s->server_list.end(),
                              random_server, server_compare);
         const SocketId id = server->id;
-        random_traversed.insert(id);
         if (0 != IsServerAvailable(id, out->ptr)) {
+            random_traversed.insert(id);
             continue;
         }
         if (!ExcludedServers::IsExcluded(in.excluded, id)) {