Sync-labels should not remove an unspecified label

[OJFord]

Description:
I think that even with sync-labels: true, a label which is not defined in labeler.yml at all should not be removed.

Justification:
Some labels might be applied by other means, such as non-labeler workflows marking a PR as 'deployed' for example, which have no file or branch based (or any static/declarative) way of describing when they should exist.

If a label is not even defined according to paths etc., then it should not be removed by labeler - we should assume it has some other source.

If people want labeler to be an absolute source of truth for allowed labels, then an ignore key or similar would do - i.e. to define a list of allowed labels which aren't synchronised.

Are you willing to submit a PR?

Willing but not that familiar.

[HarithaVattikuti]

Hello @OJFord
Thank you for creating this feature. We will investigate it and get back to you as soon as we have some feedback.

[abhishekrb19]

This feature would be a neat addition! If there's a way to differentiate manually added labels from auto-added labels by the labeler, that would be great.

With sync-labels: true, I'd expect the labeler to only sync labels that it had added. We found that the labeler would also spuriously remove labels manually added by maintainers. As a result of that, we set sync-labels: false, so we're having to manually remove any stale labels if needed.