Keda prometheus scaler tutorial (kyma-project#15305)

* improve docs

* improve docs v2

* Apply suggestions from code review

Co-authored-by: Krzysztof Kwiatosz <[email protected]>

* Fix docs

* Update svls-15-use-external-scalers.md

* Apply suggestions from code review

Co-authored-by: Grzegorz Karaluch <[email protected]>

Co-authored-by: Krzysztof Kwiatosz <[email protected]>
Co-authored-by: Grzegorz Karaluch <[email protected]>

Author: 3 people

docs/03-tutorials/00-serverless/svls-15-use-external-scalers.md

@@ -66,8 +66,8 @@ Follow these steps:
 
   </details>
   <details>
-  <summary label="keda">
-  Keda
+  <summary label="keda-cpu">
+  Keda CPU
   </summary>
 
 1. Install [Keda](https://keda.sh/docs/2.8/deploy/) if it is not present on your cluster.
@@ -118,9 +118,9 @@ Follow these steps:
     EOF
     ```
 
-    >**NOTE:** in this tutorial we use the `cpu` trigger because of its simple configuration. If you want to use another trigger check the official [list of supported triggers](https://keda.sh/docs/2.8/scalers/).
+    >**NOTE:** This tutorial uses the `cpu` trigger because of its simple configuration. If you want to use another trigger check the official [list of supported triggers](https://keda.sh/docs/2.8/scalers/).
 
-4. After a few seconds ScaledObject should be up to date and contain information about actual replicas:
+4. After a few seconds ScaledObject should be up to date and contain information about the actual replicas:
 
     ```bash
     kubectl get scaledobject scaled-function
@@ -133,5 +133,76 @@ Follow these steps:
     scaled-function   serverless.kyma-project.io/v1alpha2.Function   scaled-function   5     10    cpu                         True    True     Unknown    4m15s
     ```
 
+  </details>
+  <details>
+  <summary label="keda-prometheus">
+  Keda Prometheus
+  </summary>
+
+1. Install [Keda](https://keda.sh/docs/2.8/deploy/) if it is not present on your cluster.
+
+2. Create your Function with the `replicas` value set to 1, to prevent the internal Serverless HPA creation:
+
+    ```yaml
+    cat <<EOF | kubectl apply -f -
+    apiVersion: serverless.kyma-project.io/v1alpha2
+    kind: Function
+    metadata:
+      name: scaled-function
+    spec:
+      runtime: nodejs14
+      replicas: 1
+      source:
+        inline:
+          dependencies: ""
+          source: |
+            module.exports = {
+              main: function(event, context) {
+                return 'Hello World!'
+              }
+            }
+    EOF
+    ```
+
+3. Create the ScaledObject resource based on the `istio_requests_total` metric, exposed by the Istio:
+
+    ```yaml
+    cat <<EOF | kubectl apply -f -
+    apiVersion: keda.sh/v1alpha1
+    kind: ScaledObject
+    metadata:
+      name: scaled-function
+    spec:
+      scaleTargetRef:
+        apiVersion:    serverless.kyma-project.io/v1alpha2
+        kind:          Function
+        name:          scaled-function
+      minReplicaCount:  1  # You can go with 0 ( scaling to zero ) in case your function is fed from messaging queue that would buffer unhandled requests or if you are fine with function downtime at cold start periods
+      maxReplicaCount:  5
+      triggers:
+      - type: prometheus
+        metadata:
+          serverAddress: http://prometheus-operated.kyma-system.svc.cluster.local:9090
+          metricName: istio_requests_total
+          query: round(sum(irate(istio_requests_total{reporter=~"source",destination_service=~"scaled-function.default.svc.cluster.local"}[2m])), 0.001)
+          threshold: '6.5'
+    EOF
+    ```
+
+    >**NOTE:** This tutorial uses the `prometheus` trigger because of its simple configuration. If you want to use another trigger check the official [list of supported triggers](https://keda.sh/docs/2.8/scalers/).
+  
+4. After a few seconds ScaledObject should be up to date and contain information about the actual replicas:
+
+    ```bash
+    kubectl get scaledobject scaled-function
+    ```
+
+    You should get a result similar to this example:
+
+    ```bash
+    NAME              SCALETARGETKIND                                SCALETARGETNAME   MIN   MAX   TRIGGERS     AUTHENTICATION   READY   ACTIVE   FALLBACK   AGE
+    scaled-function   serverless.kyma-project.io/v1alpha2.Function   scaled-function   1     5     prometheus                    True    True     Unknown      4m15s
+    ```
+
 </details>
 </div>