Designates a method to be called only once, just after the end of the
benchmark run. It is always called from global thread 0 for the given driver.
The driver will wait for all threads to terminate before calling the
OnceAfter method. Note that only one method is allowed for the OnceAfter
designation. If Faban finds more than one method with the OnceBefore
annotation, it will throw a DefinitionException at startup.
The OnceBefore and OnceAfter annotations are used mainly for auditing
the state of the SUT before and after the benchmark run. A CustomMetrics
shall be used to report the audit results.