@Nullable
        @Override
        protected String propagationField(String keyName) {
            return ctx.request().headers().get(keyName);
        }
Wraps {@link ServiceRequestContext} in an {@link brave.rpc.RpcServerRequest}, for use in
{@link RpcServerHandler}.
brave/brave6/src/main/java/com/linecorp/armeria/server/brave/RpcServiceRequestContextAdapter.java