code: def _next_timestamp(self, now, last):
          
          if now > last:
              self.last = now
              return now
          else:
              self._maybe_warn(now=now)
              self.last = last + 1
              return self.last
docs: Returns the timestamp that should be used if ``now`` is the current
          time and ``last`` is the last timestamp returned by this object.
          Intended for internal and testing use only; to generate timestamps,
          call an instantiated ``MonotonicTimestampGenerator`` object.
  
          :param int now: an integer to be used as the current time, typically
              representing the current time in microseconds since the UNIX epoch
          :param int last: an integer representing the last timestamp returned by
              this object