Custom JSON output formatting of properties during serialization with the JsonFormat serializer

[leksmano]

What language does this apply to?
proto3

Describe the problem you are trying to solve.
Trying to introduce custom json output formatting for specific types that are autogenerated from protobuf definitions using the JsonFormat class

Currently only default serialization is possible, not allowing any custom printers to be added.

Describe the solution you'd like
ability to add additional custom printers for types that would allow custom Json output value formatting

Example of printers being added in the code currently:
https://github.com/protocolbuffers/protobuf/blob/main/java/util/src/main/java/com/google/protobuf/util/JsonFormat.java#L750

And the wellKnownTypePrinters is private which does not allow extensions:
https://github.com/protocolbuffers/protobuf/blob/main/java/util/src/main/java/com/google/protobuf/util/JsonFormat.java#L747

It would be helpful to allow serialization of enums according to custom rules. Currently it appears this is fixed and no override is possible:
https://github.com/protocolbuffers/protobuf/blob/main/java/util/src/main/java/com/google/protobuf/util/JsonFormat.java#L1234

Describe alternatives you've considered
We have used similar functionality with the scalpb-json4s library that allows us to override the printer for certain types using the DefaultRegistry

[JasonLunn]

It is not clear from your question how customization of JSON serialization would work given the need for the JSON to be parseable back into a Proto by a client in another language, or even within Java if they have a different set of customizations. Whereas text formatting is fairly customizable, it is not intended for exchange. JSON has to be able to roundtrip.

Maybe you could provide an example of the kind of customization you need?