如何向Appengine的webapp框架添加中间件？

pip install google-cloud-sdk

gcloud app create

import webapp2

class MainHandler(webapp2.RequestHandler):
    def get(self):
        self.response.write('Hello, world!')

app = webapp2.WSGIApplication([
    ('/', MainHandler)
], debug=True)

class LoggingMiddleware(webapp2.RequestHandler):
    def dispatch(self):
        # Before request
        start_time = time.time()
        logging.info('Started request')

        # Call the actual request handler
        super(LoggingMiddleware, self).dispatch()

        # After request
        end_time = time.time()
        logging.info('Finished request in %s seconds', end_time - start_time)

class MainHandler(LoggingMiddleware):
    def get(self):
        self.response.write('Hello, world!')

app = webapp2.WSGIApplication([
    ('/', MainHandler)
], debug=True)