* Protobuf Modernization
Remove direct usage of golang/protobuf in favor of google.golang.org/protobuf
Marshallers (protobuf and json) needed some changes to account for different APIs.
Moved to using the google.golang.org/protobuf/types/known/* for the well known types including replacing some custom Struct manipulation with whats available in the structpb well known type package.
This also updates our devtools script to install protoc-gen-go from the right location so that files it generates conform to the correct interfaces.
* Fix go-mod-tidy make target to work on all modules
* 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
Matt Keeler
Paul Glass