MINOR: contrib/prometheus-exporter: export build_info

MINOR: contrib/prometheus-exporter: export build_info

commit c55a626217a7e676e1cc ("MINOR: contrib/prometheus-exporter: Add
missing global and per-server metrics") is renaming two metrics between
v2.2 and v2.3:
  server_idle_connections_current
  server_idle_connections_limit

It is breaking some tools which are making use of those metrics while
supporting several haproxy versions. This build_info will permit tools
which make use of metrics to be able to match the haproxy version and
change the list of expected metrics. This was possible using the haproxy
stats socket but not with prometheus export.

This patch follows prometheus best pratices to export specific software
informations. It is adding a new field `build_info` so we can extend it
to other parameters if needed in the future.

example output:
  # HELP haproxy_process_build_info HAProxy build info.
  # TYPE haproxy_process_build_info gauge
  haproxy_process_build_info{version="2.4-dev5-2e1a3f-5"} 1

Even though it is not a bugfix, this patch will make more sense when
backported up to >= 2.0

Signed-off-by: William Dauchy <wdauchy@gmail.com>