type mdExtraKey … // WithExtraMetadata creates a new context with incoming md attached. func WithExtraMetadata(ctx context.Context, md metadata.MD) context.Context { … } // ExtraMetadata returns the incoming metadata in ctx if it exists. The // returned MD should not be modified. Writing to it may cause races. // Modification should be made to copies of the returned MD. func ExtraMetadata(ctx context.Context) (md metadata.MD, ok bool) { … }