MINOR: promex: Add support for specialized front/back/li/srv metric names

MINOR: promex: Add support for specialized front/back/li/srv metric names

Depending on the scope, metrics can have different names. For instance, the
number of active/backend servers are reported with
"haproxy_backend_active_servers"/"haproxy_backend_backup_servers" metric names
in the backend scope while it should be
"haproxy_server_active"/"haproxy_server_backup" in the server scope.

To be able to support different names depending on the scope for the same
metric, arrays of ISTs were added, one by scope (front, back, listen,
server). These arrays only contain names overriding the default ones.

Note: the exemple above is not supported for now and is the reason for this
commit.