Add docs for additional metrics provided by metrics server

[BojanZelic]

Metrics server actually exposes an additional metric endpoint that many probably are not aware of; This is where SLO metrics can be retrieved from;

exposes metrics like:

apiserver_request_total{group="external.metrics.k8s.io"}
apiserver_request_sli_duration_seconds_bucket{group="external.metrics.k8s.io"}

Checklist

• Commits are signed with Developer Certificate of Origin (DCO)

Fixes #4460

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[JorTurFer]

What KEDA version are you using? I'm using main tag and I don't see those metrics from metrics server prometheus endpoint

[BojanZelic]

@JorTurFer I'm using the latest 2.11, but it should be present in pretty much all versions of keda metrics server;

kubectl create serviceaccount -n default monitoring-user

kubectl apply -f - <<EOF
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  name: prometheus
rules:
  - nonResourceURLs:
      - "/metrics"
    verbs:
      - get
EOF

kubectl create clusterrolebinding prometheus --clusterrole=prometheus --serviceaccount=default:monitoring-user

kubectl port-forward -n keda keda-metrics-apiserver-66c887bbb4-5znvl 6443:6443

# new terminal
TOKEN=$(kubectl create token monitoring-user)
curl --verbose -k -H "Authorization: Bearer $TOKEN" https://localhost:6443/metrics