use `stats_fill_sv_stats` when possible to avoid duplicating code.
the following metrics have a change of behaviour:
haproxy_server_limit_sessions
haproxy_server_queue_limit
haproxy_server_check_failures_total
haproxy_server_check_up_down_total
haproxy_server_downtime_seconds_total
haproxy_server_current_throttle
haproxy_server_idle_connections_limit
depending on cases, if the limit was not configured or enabled, NaN is
returned instead. It should not be an issue for users, even better than
before as it provides more precise info.
Signed-off-by: William Dauchy <wdauchy@gmail.com>