MINOR: stats: convert req_tot as generic column

MINOR: stats: convert req_tot as generic column

req_tot counter is a special case as it is not managed identically
between frontend and backend side.

For the backend side, this metric is available directly into
be_counters, which allows to use a generic stat column definition.

On the frontend side however, the metric value is an aggredate of
multiple fe_counters value. This is the case since the splitting between
HTTP version introduced in the following patch :

  9969adbcdc1a79a6e8bb0a6283191d8d330a04f1
  MINOR: stats: add by HTTP version cumulated number of sessions and requests

This difference cannot be handled automatically by me_generate_field().
Add a special case in the function to produce it on frontend side
reusing the aggregated value. This not done however for stats-file as
there is no counter to preload.