Seeking help for "req.body" logging (no replies)

Hello,

I am currently testing 1.6-dev5 in my production environment (giving it
only 10% of the traffic), I am having some problems understanding how to
use
https://cbonte.github.io/haproxy-dconv/configuration-1.6.html#7.3.6-req.body,
can it be used to log the body of a request?

I am using it like this in both my HTTP and HTTPS frontends:

option http-buffer-request
log-format "%[req.body]"

The error I get is "'log-format' : sample fetch <req.body_len> may not be
reliably used here because it needs 'HTTP request headers' which is not
available here.", where should I be using it?
Does that mean that we cannot log req.body at all or that I have to enable
another option before trying to use it?

Any hint or help is much appreciated.
Thank you.

Cheers

Alberto