feat: validate fallback configuration

[guopeng0]

#5515

fallback feature only supports AverageValue metrics type and it doesn't support CPU and Memory.
To prevent misconfigurations and confusions related with these limitations, we should include a validation for them during the admission process

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 our Helm chart (repo) (if applicable, ie. when deployment manifests are modified)
• A PR is opened to update the documentation on (repo) (if applicable)
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

Fixes #5515

Relates to #

[JorTurFer]

Looking good! ❤️
Could you add a test for this case? We need to update the changelog as well :)

[guopeng0]

Sure, I agree. We’ll add a test for this case.

[JorTurFer]

Looks nice! Could you update docs as well to include this new rule? https://github.com/kedacore/keda-docs/blob/main/content/docs/2.14/concepts/admission-webhooks.md

[guopeng0]

Sounds good, I will update the documentation accordingly.

[guopeng0]

Hi @JorTurFer
I've addressed the issue and updated the documentation to include the new rule.

[zroubalik]

/run-e2e fallback
Update: You can check the progress here

[zroubalik]

@guopeng0 we should also check that both Replicas and FailureThreshold are set if Fallback is specified.

[guopeng0]

Hi @zroubalik
I appreciate your comment. The code considers non-negative values for Replicas and FailureThreshold as valid, and if they are not explicitly set by the user, default processing will apply with values set to 0. Thus, I believe no additional checks are required when fallback is specified.

func validateFallback(scaledObject *kedav1alpha1.ScaledObject) bool {
	return scaledObject.Spec.Fallback.FailureThreshold >= 0 &&
		scaledObject.Spec.Fallback.Replicas >= 0
}

The relevant code is located in the keda/pkg/fallback/fallback.go

[zroubalik]

@guopeng0 could you please fix the conflict in the Changelog? then we are good to go, thanks!

[guopeng0]

@zroubalik
Sure, I’ve resolved the conflict in the Changelog.

[tomkerkhove]

/run-e2e fallback
Update: You can check the progress here

[tomkerkhove]

@guopeng0 Can you fix the linting issue please?

[guopeng0]

@tomkerkhove Sure, I’ll make the changes as soon as possible.