Add an example for grpc servers

[vtolstov]

did you mind to add example for grpc server upgrade?

[lmb]

I don't have much experience with gRPC, so I can't promise anything. If you'd like to contribute one that'd be great!

[sagikazarmark]

The relevant parts of a gRPC example would be the same as the HTTP server example (most of the code would be gRPC server bootstrapping), so I'm not sure I see the value in adding a gRPC example.

[hustuhao]

Hi, everyone.I am working on this issue.