code: @Override
      public JsonElement serialize(Label label, Type type, JsonSerializationContext context)
      {
          JsonElement element = gson.toJsonTree(label, type);
          JsonObject obj = new JsonObject();
          obj.add("label", element);
          return obj;
      }
docs: Gson invokes this call-back method during serialization when it encounters a field of the specified type.
  @param label The label being serialized
  @param type The type of the Object to deserialize to
  @param context The JSON serialization context
  @return The JSON data that was serialized