MEDIUM: stats: allow to select one field in `stats_fill_fe_stats`

MEDIUM: stats: allow to select one field in `stats_fill_fe_stats`

prometheus approach requires to output all values for a given metric
name; meaning we iterate through all metrics, and then iterate in the
inner loop on all objects for this metric.
In order to allow more code reuse, adapt the stats API to be able to
select one field or fill them all otherwise.
From this patch it should be possible to remove most of the duplicate
code on prometheuse side for the frontend.

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