Wrong avg_response_time metric

Hello,

you'd better calculate the average_response_time yourselves, as uWSGI fails to provide a correct implementation of average (and they're not willing to fix it).

See [here](https://github.com/unbit/uwsgi/blob/master/core/utils.c#L1085) for the wrong code and [here](https://github.com/unbit/uwsgi/pull/730) for their position on the matter.
