code: public EntityManager createEntityManager(String projectId, String jsonCredentialsFile) {
      return createEntityManager(projectId, jsonCredentialsFile, null);
    }
docs: Creates and return a new {@link EntityManager} using the provided JSON formatted credentials.
  
  @param projectId
  the project ID
  
  @param jsonCredentialsFile
  the JSON formatted credentials file for the target Cloud project.
  @return a new {@link EntityManager}