feat: add fallback support for value metric type

[y-rabie]

• Since the constraint on having fallback only for AverageValue seems to me kinda unwarranted, it's here relaxed a bit, and can be removed altogether if we opt for another implementation.

  The constraint now becomes that the scaleTargetRef object has a field .status.readyReplicas that its controller updates with the number of ready replicas, so that we can directly use that. This is de facto the case with Deployments/StatefulSets/Replicasets/Argo Rollouts.

  We can then generically fetch the object as unstructured and access the value of the field to divide by it. A brief math illustration starting with the HPA's equation

  desiredReplicas = ceil [currentReplicas * (currentMetricValue/desiredMetricValue) ]

  By passing currentMetricValue = desiredMetricValue * fallbackReplicas / currentReplicas

  We end up with

  desiredReplicas = ceil [currentReplicas * (( desiredMetricValue * fallbackReplicas / currentReplicas )/desiredMetricValue) ]

  desiredReplicas = ceil [currentReplicas * (fallbackReplicas / currentReplicas ) ] = ceil [fallbackReplicas] = fallbackReplicas

  Emphasis: currentReplicas in HPA's equation is the number of ready replicas.

  I preferred this approach to the other one (which would remove the .status.readyReplicas field constraint) which is manually counting the number of ready pods (similar to what HPA does here), since it'd be quite involved. If it seems a better approach to you, I can implement it.

• For full clarity, a problematic nit with this: is that we're dependent on the object's controller updating the .status.readyReplicas in a timely manner.

  If there had been a lag, then the currentReplicas the HPA multiplies by (which it gets by manually counting pods) would deviate from the currentReplicas value we divide by.

  If ours is less, then we'd scale higher than fallbackReplicas for a brief while; if ours is more, then we'd scale less than fallbackReplicas. Either way we should eventually stabilize at fallbackReplicas exactly.

• Final unrelated small change for correctness sake (that I think should be fixed regardless of this PR), the line

  Value: *resource.NewMilliQuantity(normalisationValue*1000*replicas, resource.DecimalSI),

  has been changed to

  Value: *resource.NewMilliQuantity(int64(normalisationValue*1000*replicas), resource.DecimalSI),

  with normalizationValue being float64 instead of int.

  This prevents early casting to int, which would discard fractions in normalizationValue early on, that would have been already rounded when multiplying by 1000 below.

  Imagine normalisationValue=2.5 and fallbackReplicas=10, previously, Value = 2*1000*10 = 20000.

  Now, Value = 2.5*1000*10 = int64(25000.0) = 2500.

  Obviously the former will cause HPA to not scale to fallbackReplicas exactly.

For the unchecked list item, if this looks good, I'll open a PR to the docs repo.

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)
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