def finish(end_time: Time.now)
      if end_micros.nil?
        self.end_micros = LightStep.micros(end_time)
      end
      tracer.finish_span(self)
      self
    end
def finish(end_time: Time.now)
      if end_micros.nil?
        self.end_micros = LightStep.micros(end_time)
      end
      tracer.finish_span(self)
      self
    end
Finish the {Span}
 @param end_time [Time] custom end time, if not now