Support default results for missing fields

[paul-sachs]

When a service doesn't respond with a requested field, we should be able to default the field to the defined default for each type.

Context

While writing integration test mocks for knit endpoints, we often just want to mock sensible defaults without having to be explicit about the values and to just ensure type safety.

[dgroux]

You are able to use Message.fromJson() for the corresponding protoc-gen-es Message class to initialise the message (as well as default fields).

[paul-sachs]

@dgroux hmm yeah maybe that would be sufficient... I'll give it a shot.

[paul-sachs]

So fromJson only works on primitive fields and doesn't seem to populate complex message fields with their defaults. So if I have a message with the following schema:

import "google/protobuf/timestamp.proto";

message SomeMessage {
 string id = 1;
 CustomMessage custom = 2;
}

message CustomMessage {
  string id = 1;
  string name = 2;
}

And you initialize something like:

const object = new SomeMessage({
  id: "something"
}).toJson({
  emitDefaultValues: true
})

The object.custom will be undefined, instead of what I'd like, which is { id: "", name: ""}.

Unfortunately this runs into the problem of recursive message structures, so we can't just recursively initialize default messages.