add activation feature for CPU/Memory scaler

[kunwooy]

Currently, the cpu & memory scaler lacks the activation feature because it delegates the scaling responsibilities to the built-in Kubernetes HPA controller. As a result, even if the scale target is currently scaled-out by the cpu/memory metric being above the threshold value, if some other Keda scalers which use External Metrics are used in conjunction with the cpu/memory scaler, it will be deactivated (and thus scaled to zero) when all other scalers using External Metrics are deactivated.

Hence, my proposal is to introduce a way to check the activation of the cpu/memory scaler. Since the scaling behavior will be handled by the HPA controller, cpu/memory scaler only needs to feed in the activation value to the scaled object controller in its GetMetricsAndActivity() method. Moreover to enable such feature, I introduce activationValue field in cpu/memory trigger's metadata.

Checklist

• When introducing a new scaler, I agree with the scaling governance policy
• I have verified that my change is according to the deprecations & breaking changes policy
• Tests have been added
• Changelog has been updated and is aligned with our changelog requirements
• A PR is opened to update the documentation on (repo) (if applicable)
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

Fixes #6057

Relates to #

[JorTurFer]

Hello
I see the use case that you are exposing, but personally I'd not check the HPA because it can have delays or missing values, if you want to do something like this, you should query directly to the metrics server IMHO.
WDYT @zroubalik @wozniakjan ?