code: public void userCompletedAction(@NonNull final String action, JSONObject metadata) {
          userCompletedAction(action, metadata, null);
      }
docs: <p>A void call to indicate that the user has performed a specific action and for that to be
  reported to the Branch API, with additional app-defined meta data to go along with that action.</p>
  
  @param action   A {@link String} value to be passed as an action that the user has carried
  out. For example "registered" or "logged in".
  @param metadata A {@link JSONObject} containing app-defined meta-data to be attached to a
  user action that has just been completed.