Logging the response body with Nginx Lua

Hi,

I have a reverse proxy that l want to log request body with which currently looks lie

log_format my_tracking '"$request" "$http_soapaction" "$http_content_type" "$request_body"';

which is logging everything wonderfully, now I want to log the response body of these requests its turning out to be a lot harder. I have been trying to use the Lua module (which I've compiled into Nginx 1.3.14) but I can't find any examples of how to do this, I'm definitely stretching my technical ability so any assistance would be greatly received.

Thanks,
Will