In Autoware measurer:

https://github.com/CPFL/ROSCH/tree/master/Measurer/

exec_time.cpp, 
void ExecTime::start_time() {
    start_time_ms = get_current_time_ms();
}
void ExecTime::end_time() {
    end_time_ms = get_current_time_ms();
    exec_time_ms = end_time_ms>start_time_ms ? end_time_ms-start_time_ms : 0;}

But with fixed-priority preemptive scheduling, the node being measured may be preempted by higher priority nodes, hence the WCET measurement may not be accurate.