Filter pids by it's username.
        :param user: string with name of system user
        :param pids: set of pids to filter
        :return: set of filtered pids

def _filter_by_user(self, user, pids):
        """
        Filter pids by it's username.
        :param user: string with name of system user
        :param pids: set of pids to filter
        :return: set of filtered pids
        """
        filtered_pids = set()
        for pid in pids:
            try:
                proc = psutil.Process(pid)
                if proc.username() == user:
                    self.log.debug("Collecting pid {} belonging to {}".format(pid, user))
                    filtered_pids.add(pid)
                else:
                    self.log.debug("Discarding pid {} not belonging to {}".format(pid, user))
            except psutil.NoSuchProcess:
                pass

        return filtered_pids