I am trying to create a Layer that tracks metrics for my gRPC service.
I'd like to follow the OTEL semantic conventions for RPC.

I having trouble implementing rpc.server.request.size as I can't seem to find a way to get the size of the message we send over the wire.

I came across fn size_hint() on body but from what I can tell its only implemented for streams? (from my testing its always SizeHint { lower: 0, upper: None })

Is there any way to get the payload size from a Layer middleware?