Feature Suggestion: Indicate disabled state of peering

I have alert set up for `bird_protocol_up`, however, I have a few intentionally disabled peerings.

As of right now, there is no way to filter these disabled peerings out by default.

I was going to suggest adding it to `bird_protocol_up`, however, seeing the issue #63, I'm not so sure anymore.

Maybe a metric called `bird_protocol_enabled` would be in the scope?

