iXHProfRuns interface for getting/saving a XHProf run.
Clients can either use the default implementation, namely XHProfRuns_Default, of this interface or define their own implementation.
get_run($run_id, $type, $run_desc)
Also, a brief description of the run is returned via the $run_desc out parameter.
save_run($xhprof_data, $type, $run_id)
The caller may optionally pass in run_id (which they promise to be unique). If a run_id is not passed in, the implementation of this method must generated a unique run id for this saved XHProf run.
Returns the run id for the saved XHProf run.