Support Interceptor Wrapping

[empijei]

For various reasons (e.g. experiments) it might be useful to wrap interceptors.

The problem is that once an interceptor gets wrapped, it will not be matched to its configurations anymore.

I propose to add the following feature:

When a Mux is constructed we match configs with their interceptor, but we also check if the the interceptor has a

Unwrap() Interceptor

method on it.

If so, we recursively check configs against the inner interceptors.

[kele]

Is my understanding correct that we don't need anything extra to support this case? Maybe just a GoDoc example?