MINOR: contrib/prometheus-exporter: use fill_be_stats for backend dump

MINOR: contrib/prometheus-exporter: use fill_be_stats for backend dump

use `stats_fill_be_stats` when possible to avoid duplicating code; make
use of field selector to get the needed field only.

the only difference is on `haproxy_backend_downtime_seconds_total` as
stats.c is testing `px->srv`. This behaviour is present since commit
7344f4789321ef8ce2ce17cf73dabd672f7c8c6 ("MINOR: stats: only report
backend's down time if it has servers"). The end result is a NaN instead
of a zero when no server are present.

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