GRPC has a reflection protocol [1] that can be used to build Swagger style tools [2]

[1] https://github.com/grpc/grpc/blob/master/doc/server-reflecti...

[2] https://github.com/grpc-swagger/grpc-swagger

You can see that reflection in action through tools like grpcui:

https://github.com/fullstorydev/grpcui