code: func AuthFromMD(ctx context.Context, expectedScheme string) (string, error) {
  	val := metautils.ExtractIncoming(ctx).Get(headerAuthorize)
  	if val == "" {
  		return "", grpc.Errorf(codes.Unauthenticated, "Request unauthenticated with "+expectedScheme)
  
  	}
  	splits := strings.SplitN(val, " ", 2)
  	if len(splits) < 2 {
  		return "", grpc.Errorf(codes.Unauthenticated, "Bad authorization string")
  	}
  	if strings.ToLower(splits[0]) != strings.ToLower(expectedScheme) {
  		return "", grpc.Errorf(codes.Unauthenticated, "Request unauthenticated with "+expectedScheme)
  	}
  	return splits[1], nil
  }
docs: // AuthFromMD is a helper function for extracting the :authorization header from the gRPC metadata of the request.
  //
  // It expects the `:authorization` header to be of a certain scheme (e.g. `basic`, `bearer`), in a
  // case-insensitive format (see rfc2617, sec 1.2). If no such authorization is found, or the token
  // is of wrong scheme, an error with gRPC status `Unauthenticated` is returned.