Add wrapper to assign an instance name

Passing of the instance name between components is currently very awkward. I'm starting a set of MRs which will reduce the need to do as much plumbing with this variable, by assigning it to a context var. This var can be used by any component needing to know the instance name, such as metrics publisher logic and other decorators like the auth.

I also am using a pattern in the MR which allows a single decorator to be used for all of unary/unary, unary/stream, stream/unary, and stream/stream request endpoints.