The grpc-gateway library that is used to transform REST calls into gRPC
uses a different method for reading a request body stream depending on
whether the RPC is a request-streaming one or not. We can't really find
out what kind of RPC the user is calling at runtime, so we add a new
parameter to the proxy that lists all request-streaming RPC calls.
In any case the client _has_ to send one request message initially to
kick off the request processing. Normally this can just be an empty
message. This can lead to problems if that empty message is not
expected by the gRPC server. But for the currently existing two
client-streaming RPCs this will only trigger a warning
(HTLC interceptor) or be ignored (channel acceptor).
