what should be the format of http logs of a proxy?

In this specific case I am using fabio and nginx and apache http proxy that just proxies requests to destinations. Both the "Combined Log Format" and "Common Log Format" _do not_ include where to a proxy request is redirected and if the client is using http or https of the request and to which host the request wants to connect to. They also do not include the client port of the request. Is there a standardized "proxy log format" that should be used to log such information? How do I go about adding them - on the end of common log format? or should I ignore and just come up with something of my log format?