grpc-trace-bin header support

[xd009642]

As well as traceparent for context propagation there's also grpc-trace-bin and x-cloud-trace-context, my current work system is using grpc-trace-bin so it would be nice if it was as easy to propagate the traces as it was with traceparent or the b3 headers

[jtescher]

The spec discussion for this is currently happening in open-telemetry/opentelemetry-specification#639

[hdost]

There's no reason this could not be implemented as an extension crate, it doesn't need to be built into the rust sdk/api.

[hdost]

So i think while this would provide some compatibility with open census, this doesn't seem like it will be in the root. We split the repos recently. I think we can transfer this to the contrib repo. Spec https://graphql-faas.github.io/opencensus-specs/trace/gRPC.html if anyone wants to implement.