[docs] Add a dedicated K8s targets page (#452)

* We'll add more examples to this page over time.

docs/content/docs/how-to/k8s_targets.md

@@ -0,0 +1,161 @@
+---
+menu:
+  docs:
+    parent: "how-to"
+    weight: 11
+title: "Kubernetes Targets"
+date: 2022-11-01T17:24:32-07:00
+---
+
+If you're running on Kubernetes, you'd probably want to monitor Kubernetes
+resources (e.g. pods, endpoints, ingresses, etc) as well. Cloudprober supports
+[dynamic discovery]({{< ref targets.md >}}#dynamically-discovered-targets) of
+Kubernetes resources through the targets type
+[`k8s`](https://github.com/cloudprober/cloudprober/blob/ad73fe489ea3ac69e7b0f81a465671df9adc8321/targets/proto/targets.proto#L40).
+
+For example, the following config adds an HTTP probe for the endpoints named
+`cloudprober` (equivalent to running _kubectl get ep cloudprober_).
+
+```bash
+probe {
+  name: "pod-to-endpoints"
+  type: HTTP
+
+  targets {
+    # Equivalent to kubectl get ep cloudprober
+    k8s {
+      endpoints: "cloudprober"
+    }
+  }
+
+  # Note that the following http_probe automatically uses target's discovered
+  # port.
+  http_probe {
+    relative_url: "/status"
+  }
+}
+```
+
+### Supported Resource and Filters
+
+Cloudprober supports discovery for the following k8s resources:
+
+- **Services**
+- **Endpoints**
+- **Pods**
+- **Ingresses**
+
+#### Filters
+
+You can filter k8s resources using the following options:
+
+- `name`: (regex) Resource name filter. It can be a regex. Example:
+  ```shell
+  # Endpoints with names ending in "service"
+  targets {
+    k8s {
+        endpoints: ".*-service"
+    }
+  }
+  ```
+- `namespace`: Namespace filter. Example:
+  ```shell
+  # Ingresses in "prod" namespace, ending in "lb"
+  targets {
+    k8s {
+        namespace: "prod"
+        ingresses: ".*-lb"
+    }
+  }
+  ```
+  ```shell
+  # Kube-DNS service
+  targets {
+    k8s {
+        namespace: "kube-system"
+        services: "kube-dns"
+    }
+  }
+  ```
+- `labelSelector`: Label based selector. It can be repeated, and works similar
+  to the kubectl's --selector/-l flag. Example:
+  ```shell
+  targets {
+    k8s {
+        pods: ".*"
+        labelSelector: "k8s-app"         # k8a-app label exists
+        labelSelector: "role=frontend"   # label "role" is set to "frontend"
+        labelSelector: "!no-monitoring"  # label "no-monitoring is not set"
+    }
+  }
+  ```
+- `portFilter`: (regex) Filter resources by port name or number (if port name is
+  not set). This is useful for resources like endpoints and services, where each
+  resource may have multiple ports. Example:
+  ```shell
+  targets {
+    k8s {
+        endpoints: ".*-service"
+        portFilter: "http-.*"
+    }
+  }
+  ```
+
+### Cluster Resources Access
+
+Note: If you've installed Cloudprober using
+[Helm Chart](https://artifacthub.io/packages/helm/cloudprober/cloudprober), this
+step is automatically taken care of.
+
+Cloudprober discovers k8s resources using kubernetes APIs. It assumes that we
+are interested in the cluster we are running it in, and uses in-cluster config
+to talk to the kubernetes API server. For this set up to work, we need to give
+our container read-only access to kubernetes resources:
+
+```yaml
+# Define a ClusterRole (resource-reader) for read-only access to the cluster
+# resources and bind this ClusterRole to the default service account.
+
+cat <<EOF | kubectl apply -f -
+apiVersion: v1
+kind: ServiceAccount
+metadata:
+  name: cloudprober
+---
+apiVersion: rbac.authorization.k8s.io/v1
+kind: ClusterRole
+metadata:
+  annotations:
+    rbac.authorization.kubernetes.io/autoupdate: "true"
+  name: resource-reader
+  namespace: default
+rules:
+- apiGroups: [""]
+  resources: ["*"]
+  verbs: ["get", "list"]
+- apiGroups:
+  - extensions
+  - "networking.k8s.io" # k8s 1.14+
+  resources:
+  - ingresses
+  - ingresses/status
+  verbs: ["get", "list"]
+---
+apiVersion: rbac.authorization.k8s.io/v1
+kind: ClusterRoleBinding
+metadata:
+ name: default-resource-reader
+ namespace: default
+subjects:
+- kind: ServiceAccount
+  name: cloudprober
+  namespace: default
+roleRef:
+ kind: ClusterRole
+ name: resource-reader
+ apiGroup: rbac.authorization.k8s.io
+EOF
+```
+
+This will create a new service account `cloudprober` and will give it read-only
+access to the cluster resources.