@Override
    public ArmeriaRetrofitBuilder rpcDecorator(DecoratingRpcClientFunction decorator) {
        return (ArmeriaRetrofitBuilder) super.rpcDecorator(decorator);
    }
Raises an {@link UnsupportedOperationException} because this builder doesn't support RPC-level but only
HTTP-level decorators.
retrofit2/src/main/java/com/linecorp/armeria/client/retrofit2/ArmeriaRetrofitBuilder.java