core/play-java/src/main/java/play/libs/Comet.java
public static Flow<JsonNode, ByteString, NotUsed> json(String callbackName) {
    return Flow.of(JsonNode.class)
        .map(
            json -> {
              return ByteString.fromString(Json.stringify(json));
            })
        .via(flow(callbackName));
  }
public static Flow<JsonNode, ByteString, NotUsed> json(String callbackName) {
    return Flow.of(JsonNode.class)
        .map(
            json -> {
              return ByteString.fromString(Json.stringify(json));
            })
        .via(flow(callbackName));
  }
Produces a flow of ByteString using `Json.stringify` from a Flow of JsonNode. Calls out to
Comet.flow internally.

@param callbackName the javascript callback method.
@return a flow of ByteString elements.