Fix bug in system time and user time calculation

test/cpp/qps/driver.cc

@@ -137,14 +137,14 @@ static void postprocess_scenario_result(ScenarioResult* result) {
 
   auto server_system_time = 100.0 *
                             sum(result->server_stats(), ServerSystemTime) /
-                            sum(result->server_stats(), ServerWallTime);
+                            average(result->server_stats(), ServerWallTime);
   auto server_user_time = 100.0 * sum(result->server_stats(), ServerUserTime) /
-                          sum(result->server_stats(), ServerWallTime);
+                          average(result->server_stats(), ServerWallTime);
 
   auto client_system_time = 100.0 * sum(result->client_stats(), SystemTime) /
-                            sum(result->client_stats(), WallTime);
+                            average(result->client_stats(), WallTime);
   auto client_user_time = 100.0 * sum(result->client_stats(), UserTime) /
-                          sum(result->client_stats(), WallTime);
+                          average(result->client_stats(), WallTime);
 
   result->mutable_summary()->set_server_system_time(server_system_time);
   result->mutable_summary()->set_server_user_time(server_user_time);