Implements the gRPC middleware for rate-limiting as a tap.ServerInHandle
function (executed before the request is unmarshaled).
Mappings between gRPC methods and their operation type are generated by
a protoc plugin introduced by #15564.
* Move stats.go from grpc-internal to grpc-middleware
* Update grpc server metrics with server type label
* Add stats test to grpc-external
* Remove global metrics instance from grpc server tests
