code: @Nullable
    public static String getProjectIdFromComputeEngine() {
      String cachedProjectId = projectIdFromComputeEngine.get();
      return cachedProjectId != null ? cachedProjectId : queryProjectIdFromComputeEngine();
    }
docs: Gets the project ID from the Compute Engine metadata server. Returns {@code null} if the
  project ID cannot be determined (because, for instance, the code is not running on Compute
  Engine).