MINOR: promex: backend aggregated server check status

MINOR: promex: backend aggregated server check status

- add new metric: `haproxy_backend_agg_server_check_status`
  it counts the number of servers matching a specific check status
  this permits to exclude per server check status as the usage is often
  to rely on the total. Indeed in large setup having thousands of
  servers per backend the memory impact is not neglible to store the per
  server metric.
- realign promex_str_metrics array

quite simple implementation - we could improve it later by adding an
internal state to the prometheus exporter, thus to avoid counting at
every dump.

this patch is an attempt to close github issue #1312. It may bebackported
to 2.4 if requested.

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