MINOR: contrib/prometheus-exporter: Add the last heathcheck duration metric

MINOR: contrib/prometheus-exporter: Add the last heathcheck duration metric

ST_F_CHECK_DURATION is now part of exported server metrics, named
haproxy_server_check_duration_seconds and expressed in seconds. For a given
server, this value is exported only if the healthcheck is finished (the status
is greater or equal to HCHK_STATUS_CHECKED).

This patch fixes the issue #519. It may be backported as fat as 2.0.

(cherry picked from commit 2711e51016cfc408deb03e66f46bea0fa5bf954c)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>