I think aspect refers to the view of the client, in this case we add the capacity to get metric information from any proxy object. The interceptor is really the class that makes it happen.
It is a great idea, let's think about it. I think Scott wants to revamp the monitoring infrastructure as we go 24x7 in large shops commercially. Please bring that up with him when it starts. We should in fact start a sub-project here.
Detyped or typed is same-same. Possibly typed would be a bit simpler to use but you would have to have them present at deployment time (i.e. you can't turn monitoring off completely, meaning *remove* it from the chain)