HAproxy user-agent access log

In some case, HAproxy work better than Nginx ,but I need to log an user-agent .

defaults
 log-format "%{+Q}o \"%{-Q}ci\" %r %hrl %ST %B %cp %ms %ft %b %s %TR %Tw %Tc %Tr %Ta %tsc %ac %fc %bc %sc %rc %sq %bq %CC %CS"

and the important part.

frontend www-https
 capture request header user-agent len 200

Output:

done