Add doc on how to use Istio Authorization with Knative and Istio mesh mode (#2583)

* Add doc on how to use Istio Authorization with Knative

* Fix nit

* Fix nit 2

* Fix nit 3

* Fix review comments

* Replace health with healthz

* Add comment for mTLS requirment

* Fix review comment

* Fix review comments

* Remove allowing list by namespace

docs/serving/istio-authorization.md

@@ -0,0 +1,65 @@
+---
+title: "Enabling requests to Knative services when additional authorization policies are enabled"
+weight: 25
+type: "docs"
+---
+
+Knative Serving system pods, such as the activator and autoscaler components, require access to your deployed Knative services.
+If you have configured additional security features, such as Istio's authorization policy, you must enable access to your Knative service for these system pods.
+
+## Before you begin
+
+You must meet the following prerequisites to use Istio AuthorizationPolicy:
+
+- [Istio must be used for your Knative Ingress](https://knative.dev/docs/install/any-kubernetes-cluster/#installing-the-serving-component).
+- [Istio sidecar injection must be enabled](https://istio.io/latest/docs/setup/additional-setup/sidecar-injection/).
+
+## Enabling Istio AuthorizationPolicy
+
+For example, the following authorization policy denies all requests to workloads in namespace `serving-tests`.
+
+```
+$ cat <<EOF | kubectl apply -f -
+apiVersion: security.istio.io/v1beta1
+kind: AuthorizationPolicy
+metadata:
+  name: deny-all
+  namespace: serving-tests
+spec:
+  {}
+EOF
+```
+
+In addition to allowing your application path, you'll need to configure Istio AuthorizationPolicy
+to allow health checking and metrics collection to your applications from system pods.
+You can allow access from system pods [by paths](#allow-access-from-system-pods-by-paths).
+
+## Allowing access from system pods by paths
+
+Knative system pods access your application using the following paths:
+
+- `/metrics`
+- `/healthz`
+
+The `/metrics` path allows the autoscaler pod to collect metrics.
+The `/healthz` path allows system pods to probe the service.
+
+You can add the `/metrics` and `/healthz` paths to the AuthorizationPolicy as shown in the example:
+
+```
+$ cat <<EOF | kubectl apply -f -
+apiVersion: security.istio.io/v1beta1
+kind: AuthorizationPolicy
+metadata:
+  name: allowlist-by-paths
+  namespace: serving-tests
+spec:
+  action: ALLOW
+  rules:
+  - to:
+    - operation:
+        paths:
+        - /metrics   # The path to collect metrics by system pod.
+        - /healthz   # The path to probe by system pod.
+EOF
+```