docs: @SuppressWarnings("WeakerAccess")
    public CreateInstanceRequest addLabel(@Nonnull String key, @Nonnull String value) {
      Preconditions.checkNotNull(key, "Key can't be null");
      Preconditions.checkNotNull(value, "Value can't be null");
      builder.getInstanceBuilder().putLabels(key, value);
      return this;
    }