MINOR: promex: Export active/backup metrics per-server

MINOR: promex: Export active/backup metrics per-server

numbers of active and backup servers per backend were exported but the info
was not exported per-server. The main issue to do so was we were unable to
have a different name for the same metric in a different scope. Thanks to
the previous patch ("MINOR: promex: Add support for specialized
front/back/li/srv metric names") it is now possible. So the info is now
exported per-server.

This patch should fix the issue #2271.