fix postgresql: measure rtt better

Tests: no
commit_hash:37881ef48ae4c7952e896db8aafbc6e35577c9a2

Author: polyashov

postgresql/src/storages/postgres/detail/topology/hot_standby.cpp

@@ -315,18 +315,17 @@ void HotStandby::RunCheck(DsnIndex idx) {
         }
     }
     auto deadline = GetTestsuiteControl().MakeExecuteDeadline(kCheckTimeout);
-    auto start = std::chrono::steady_clock::now();
     try {
         state.connection->RefreshReplicaState(deadline);
         state.role = state.connection->IsInRecovery() ? ClusterHostType::kSlave : ClusterHostType::kMaster;
         state.is_readonly = state.connection->IsReadOnly();
-        state.roundtrip_time = std::chrono::duration_cast<Rtt>(std::chrono::steady_clock::now() - start);
-
         const auto& wal_info_stmts = GetWalInfoStatementsForVersion(state.connection->GetServerVersion());
         std::optional<std::chrono::system_clock::time_point> current_xact_timestamp;
 
+        auto start = std::chrono::steady_clock::now();
         const auto wal_info =
             state.connection->Execute(state.connection->IsInRecovery() ? wal_info_stmts.slave : wal_info_stmts.master);
+        state.roundtrip_time = std::chrono::duration_cast<Rtt>(std::chrono::steady_clock::now() - start);
         wal_info.Front().To(state.wal_lsn, current_xact_timestamp);
         if (current_xact_timestamp) {
             state.current_xact_timestamp = *current_xact_timestamp;