"haproxy_backend_agg_server_status" and "haproxy_backend_agg_check_status"
were not referenced in promex README.
"haproxy_backend_agg_server_check_status" is also missing but it is a
deprecated metric. Thus, it is better to not reference it.
Let's start to better organize the addons by moving promex there (and
with an easier directory name). The makefile and maintainers files were
updated, as well as the CI's build matrix.