Conditional access log formatting ?

[Ben]

Hi,

I'm using haproxy infront of some NGINX instances, but only for some 
instances which are being loadbalanced by haproxy, there are other 
instances which go straight through.

I realise I need to change the logging format in NGINX to enable 
X-Forwarded-For in the access logs.

The question is whether I can conditionally format the logs, i.e. use 
"X-Forwarded-For" if it exists, otherwise use the normal NGINX logging 
format.

I've had a quick peek at the manual, I'm thinking maybe this can be done 
with a map of some sort ?  Maybe there's a better way ?  To be honest 
I've no idea where to start, and I've never used maps before anyway !