AFAIK, in Google's case the end application doesn't really see the "GRPC HTTP headers" but instead they convert an incoming HTTP request to one of their frontends and route it to multiple backends (sending effectively the HTTP body serialized via GRPC), and then the frontends will simply re-create the response by unifying them.

Not a Googler, this is what I understood from reading similar things over multiple posts. Feel free to correct me.

The underlying protocol is not necessarily HTTP.