code: func Trailer(md *metadata.MD) *CallOption {
  	return &CallOption{
  		grpc.Trailer(md),
  		func(o *Options) {
  			o.resTrailerMetadata = md
  		},
  	}
  }
docs: // Trailer returns a CallOption that retrieves the trailer metadata.
  // Can be used instead of grpc.Trailer.