Feature request: grpc conn manager

Ref: https://github.com/go-kit/kit/blob/master/examples/apigateway/main.go#L184

Based on this hint, I wrote some code and wondered if this could be contributed to the kit. Do you have time to look at it @ChrisHines ? If it is not good enough, can it be an example? Here, https://github.com/MaruHyl/xgrpc