MINOR: contrib/prometheus-exporter: Add missing global and per-server metrics

MINOR: contrib/prometheus-exporter: Add missing global and per-server metrics

Following metrics are now exported by the prometheus exporter to reflect recent
changes on HAProxy :

  * haproxy_process_failed_resolutions
  * haproxy_process_bytes_out_total
  * haproxy_process_spliced_bytes_out_total
  * haproxy_process_bytes_out_rate

and

 * haproxy_server_unsafe_idle_connections_current
 * haproxy_server_safe_idle_connections_current
 * haproxy_server_used_connections_current
 * haproxy_server_need_connections_current