MINOR: contrib/prometheus-exporter: Report metrics about max times for sessions

MINOR: contrib/prometheus-exporter: Report metrics about max times for sessions

Now, for the sessions, the maximum times (queue, connect, response, total) are
reported in addition of the averages over the last 1024 connections. These
metrics are reported per backend and per server. Here are the metrics name :

  * haproxy_backend_max_queue_time_seconds
  * haproxy_backend_max_connect_time_seconds
  * haproxy_backend_max_response_time_seconds
  * haproxy_backend_max_total_time_seconds

and

  * haproxy_server_max_queue_time_seconds
  * haproxy_server_max_connect_time_seconds
  * haproxy_server_max_response_time_seconds
  * haproxy_server_max_total_time_seconds

This patch is related to #272.