Restrict Default self-hosted Runner Labels

[Kerruba]

Currently, it is possible to add custom self-hosted runner labels to the runner-label checker using the following configuration:

self-hosted-runner:
    labels:
      - private-runner-label1
      - private-runner-label2

By default, the self-hosted label is considered a valid option for runners.

However, when using the Actions Runner Controller (ARC) to manage self-hosted runners on Kubernetes, it is not possible to assign multiple labels to the runner-set. Additionally, the default self-hosted label is not automatically added to the runner group in this setup.

As a result, the default self-hosted label often becomes an invalid option in such scenarios.

Proposal

It would be highly beneficial to provide an option in the configuration to restrict valid self-hosted runner labels to only those explicitly defined by the user.

For example, a new field could be introduced to disable the default acceptance of the self-hosted label unless explicitly included in the user-provided list.

This feature would improve compatibility with ARC and similar setups, ensuring labels are validated based on the user’s intended configuration.

[wyardley]

An interesting additional question is how things should behave with these third party hosted runner services like Blacksmith, Warpbuild, etc.

So far, I've been explicitly adding all of their versions explicitly under self-hosted-runner.labels, but not sure if there should be some builtin config presets for these, or a different config option?