Fix APA bugs in creation, add test and demo yaml

[kr11]

Pull Request Description

[Please provide a clear and concise description of your changes here]

This PR addresses several some issues related to APA:

1. Bug Fix: Resolves the panic encountered during the creation of APA resources. The panic was due to unimplementation of GetUpFluctuationTolerance and GetDownFluctuationTolerance.

2. New Configuration: Enables users to define a custom window for APA through the context attribute apa.autoscaling.aibrix.ai/window, the duration of the metrics window that APA observes.

3. Testing: Adds APA UT.

Demo PA yaml

Demo APA yaml:

apiVersion: autoscaling.aibrix.ai/v1alpha1
kind: PodAutoscaler
metadata:
  name: metric-server-autoscaler
  namespace: kube-system
  labels:
    app.kubernetes.io/name: aibrix
    app.kubernetes.io/managed-by: kustomize
    autoscaling.aibrix.ai/up-fluctuation-tolerance: "0.1"
    autoscaling.aibrix.ai/down-fluctuation-tolerance: "0.2"
    apa.autoscaling.aibrix.ai/window: "30s"
  namespace: default
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: metrics-server
  minReplicas: 1
  maxReplicas: 4
  metricsSources:
  - metricSourceType: "pod"
    protocolType: "https"
    port: "4443"
    path: "/metrics"
    targetMetric: "go_threads"
    targetValue: "20"
  scalingStrategy: "APA"

Demo HPA yaml:

apiVersion: autoscaling.aibrix.ai/v1alpha1
kind: PodAutoscaler
metadata:
  name: metric-server-autoscaler
  namespace: kube-system
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: metrics-server
  minReplicas: 1
  maxReplicas: 4
  metricsSources:
    - metricSourceType: "pod"
      protocolType: "https"
      port: "4443"
      path: "/metrics"
      targetMetric: "go_threads"
      targetValue: "20"
  scalingStrategy: "HPA"

The created HPA description:

kubectl describe hpa -n kube-system
Name:                    metric-server-autoscaler
Namespace:               kube-system
Labels:                  <none>
Annotations:             <none>
CreationTimestamp:       Tue, 17 Dec 2024 21:27:38 +0800
Reference:               Deployment/metrics-server
Metrics:                 ( current / target )
  "go_threads" on pods:  <unknown> / 20
Min replicas:            1
Max replicas:            4
Deployment pods:         1 current / 0 desired
Conditions:
  Type           Status  Reason               Message
  ----           ------  ------               -------
  AbleToScale    True    SucceededGetScale    the HPA controller was able to get the target's current scale
  ScalingActive  False   FailedGetPodsMetric  the HPA was unable to compute the replica count: unable to get metric go_threads: unable to fetch metrics from custom metrics API: no custom metrics API (custom.metrics.k8s.io) registered
Events:
  Type     Reason                        Age   From                       Message
  ----     ------                        ----  ----                       -------
  Warning  FailedGetPodsMetric           12s   horizontal-pod-autoscaler  unable to get metric go_threads: unable to fetch metrics from custom metrics API: no custom metrics API (custom.metrics.k8s.io) registered
  Warning  FailedComputeMetricsReplicas  12s   horizontal-pod-autoscaler  invalid metrics (1 invalid out of 1), first error is: failed to get pods metric value: unable to get metric go_threads: unable to fetch metrics from custom metrics API: no custom metrics API (custom.metrics.k8s.io) registered

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[Jeffwan]

/lgtm
rerun the test and it pass. we can merge the issue now.