Custom transports cannot use call options or exchange response headers and trailers for unary methods

[Joshua Humphries]

gRPC's call options, which are passed to stub interfaces, are opaque. This prevents a custom transport implementation from examining them and properly acting on them.

In particular, this means that a unary method invocation cannot get response headers since that is done via call options. There is an issue tracking this API deficiency in the grpc-go repo: grpc/grpc-go#1495

There is a similar issue in the server-side. Attempts to send response headers from unary handlers that are invoked from a custom transport implementation will always return an error. This is because the grpc.SetHeader (and company) methods are expecting to find a *transport.Stream in context (which is put there by the HTTP/2 transport implementation). But no API is exposed to allow custom transports to do something similar. This issue is also being tracked in the grpc-go repo: grpc/grpc-go#1802

This grpchan bug will persist until these issues are addressed.

I happen to have two open pull requests which would address them. I am hoping they will be accepted:
grpc/grpc-go#1902 and grpc/grpc-go#1904.