[IMPORTANT] Move this repository to go-grpc-middleware.v2

[bwplotka]

Hi,

As maintainers, we decided it would be healthy and much easier to maintain and improve this project if we would join the effort on creating best middleware (interceptors) together with go-grpc-middleware project. In fact, I am maintaining both of the projects. (:

We believe it can increase movement and allows us to clean up old dead code that we cannot remove due to compatibility guarantees of v1.

We propose the following:

• Migrate prometheus middleware as a new metric interceptor for newly rewritten go-grpc-middleware.v2 as designed in go-grpc-middleware.v2: Action Plan. go-grpc-middleware#275
• Document it there.
• Put this repository in the maintenance mode and support v1 bugfixes only for some time (not sure how long). All improvements and features should go to go-grpc-middleware.v2

Help wanted for initial implementation of Prometheus interceptor inside go-grpc-middleware.v2 🤗

cc @brancz