code: @BetaApi
    public static FieldValue of(Attribute attribute, Object value) {
      return new FieldValue(attribute, value);
    }
docs: Creates an instance of {@code FieldValue}, useful for testing.
  
  <p>If the {@code attribute} is {@link Attribute#PRIMITIVE}, the {@code value} should be the
  string representation of the underlying value, eg {@code "123"} for number {@code 123}.
  
  <p>If the {@code attribute} is {@link Attribute#REPEATED} or {@link Attribute#RECORD}, the
  {@code value} should be {@code List} of {@link FieldValue}s or {@link FieldValueList},
  respectively.
  
  <p>This method is unstable. See <a
  href="https://github.com/googleapis/google-cloud-java/pull/2891">this discussion</a> for more
  context.