MEDIUM: promex/resolvers: Dump resolvers metrics via a promex module

MEDIUM: promex/resolvers: Dump resolvers metrics via a promex module

Just like for stick-tables, this patch adds a promex module to dump
resolvers metrics. It adds the "resolver" scope and for now, it dumps
folloowing metrics:

  * haproxy_resolver_sent
  * haproxy_resolver_send_error
  * haproxy_resolver_valid
  * haproxy_resolver_update
  * haproxy_resolver_cname
  * haproxy_resolver_cname_error
  * haproxy_resolver_any_err
  * haproxy_resolver_nx
  * haproxy_resolver_timeout
  * haproxy_resolver_refused
  * haproxy_resolver_other
  * haproxy_resolver_invalid
  * haproxy_resolver_too_big
  * haproxy_resolver_outdated