Serialization fields order

I have an environment in which the order of the elements serialized is important.
I expected built_value to serialize fields in the order in which they are defined.
But I have seen that first non nullable field are listed and the nullable ones.
Is it possible to change this behavior?
I thought in creating a plugin for reordering fields, but I couldn't find a way to discover the original declaration order.
Finally I created my custom serializer, simply taking the standard one and changing the order in which the field are defined.
This works, but creating custom serializer for all classes is a big work (I just did for the main one), and I'd like to have the less custom serializers that I can.
Any hint?