MINOR: stats: Report max times in addition of the averages 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
values are called qtime_max, ctime_max, rtime_max and ttime_max.

This patch is related to #272.

(cherry picked from commit 0d1c2a65e8370a770d0160074f0d505a6014250e)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>

[Cf: Forgot to backport this patch in 2.0. It is required to build the
     Prometheus exporter]
2 files changed