RFR: 2591: Improve auto labeler [v2]

[Erik Joelsson]

On Wed, 24 Sep 2025 17:46:30 GMT, Zhao Song <zsong at openjdk.org> wrote:

>> After [SKARA-2588](https://bugs.openjdk.org/browse/SKARA-2588) is deployed, users are complaining about the behavior of auto labeler.
>> 
>> After discussion with Erik, we propose the following behavior for the auto labeler:
>>  1. The auto labeler will never remove labels that have already been added.
>>  2. The only case in which the auto labeler upgrades labels to group labels is when it detects a bunch of new labels to add. In this case, it will attempt to update the labels to group labels.
>>  3. Users can add any labels they want.
>>  
>>  Example:
>> 1. In the initial labeling, the auto labeler adds “hotspot-runtime” to the PR.
>>       (Current labels: “hotspot-runtime”)
>> 2. The user pushes three commits that touch “hotspot-gc” and “hotspot-jfr”. The auto labeler upgrades these to the group label “hotspot” and adds it to the PR.
>>      (Current labels: “hotspot-runtime”, “hotspot”)
>> 3. Later, the user pushes a new commit touches only “hotspot-gc”. The auto labeler will not add “hotspot-gc” to the PR since “hotspot” label is already there.
>>      (Current labels: “hotspot-runtime”, “hotspot”)
>
> Zhao Song has updated the pull request incrementally with one additional commit since the last revision:
> 
>   review comment

Marked as reviewed by erikj (Lead).

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PR Review: https://git.openjdk.org/skara/pull/1741#pullrequestreview-3273345137