docs: 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