CPU & memory resource usage not measured when `set_pid` is used

[alahiff]

Version tested: 2.0.0a1

[wk9874]

This will probably be because set_pid updates self._pid, but self._parent_process, which is used in the generation of the lists of processes to collect stats from, is only set equal to self._pid inside run.init(). To fix, just going to add a self._parent_process = psutil.Process(self._pid) into set_pid