Add option including JSON default value for transcoding

[mutexd]

Describe the feature

The nice thing about grpc-transcoding is that one can have one grpc-service implementation which can serve both Grpc/REST entry point. However, the JsonPrinter's default behavior excludes default values such as int32:0, string:"". Therefore, a GrpcResponse with
{ "message": "Hello", "status": 0 }
will become like this for REST response.
{ "message": "Hello" }
Even though it's valid in terms of ProtoJSON, it would be nice to have default values because JSON in REST conventionally includes default value like this.

Suggestion: Add an option in build time or even run time for including JsonDefaultValue in grpc-transcoding

Contribution

No response

[zZHorizonZz]

Thank you for opening the issue.

I discussed this with @vietj and we think this would be indeed userful, only thing now is that it needs more discussed in terms of working and implementation. I think we should allow for simillar stuff like grpc-gateway does. The general idea would be this being runtime as json format is not necessarily made just for transcoding.

We just need to figure out how to actually make this work.

[zZHorizonZz]

So, we have done some thinking and we think that WireFormat should perhaps be converted from enum to interface and then we should be able to pass configuration and stuff trough the wireformat. I will do some experimenting and will see with what I can come up.