MINOR: stats: duplicate 3 fields in bytes in info

MINOR: stats: duplicate 3 fields in bytes in info

in order to prepare a possible merge of fields between haproxy stats and
prometheus, duplicate 3 fields:
  INF_MEMMAX
  INF_POOL_ALLOC
  INF_POOL_USED
Those were specifically named in MB unit which is not what prometheus
recommends. We therefore used them but changed the unit while doing the
calculation. It created a specific case for that, up to the description.
This patch:
- removes some possible confusion, i.e. using MB field for bytes
- will permit an easier merge of fields such as description

First consequence for now, is that we can remove the calculation on
prometheus side and move it on `fill_info`.

Signed-off-by: William Dauchy <wdauchy@gmail.com>