docs: describe automatic application discovery (#1304)

Signed-off-by: Florian Bacher <florian.bacher@dynatrace.com>

docs/content/en/docs/concepts/apps/_index.md

@@ -32,3 +32,96 @@ spec:
 
 While changes in the workload version will affect only workload checks, a change in the app version will also cause a
 new execution of app level checks.
+
+## Automatic App Discovery
+
+The Keptn Lifecycle Toolkit also provides the option to automatically discover `KeptnApp`s, based on the
+recommended Kubernetes labels `app.kubernetes.io/part-of`, `app.kubernetes.io/name` `app.kubernetes.io/version`.
+This allows users to enable the observability features provided by the Lifecycle Toolkit for
+their existing applications, without the need for creating any Keptn-related custom resources.
+
+To enable the automatic discovery of `KeptnApp`s for your existing applications, the following steps will
+be required:
+
+1. Make sure the namespace of your application is enabled to be managed by the Keptn Lifecycle Toolkit,
+by adding the annotation `keptn.sh/lifecycle-toolkit: "enabled"` to your namespace.
+2. Make sure the following labels and/or annotations are present in the pod template
+specs of your Workloads (i.e. `Deployments`/`StatefulSets`/`DaemonSets`/`ReplicaSets`) within your application:
+    - `app.kubernetes.io/name`: Determines the name of the generated `KeptnWorkload` representing the
+    Workload.
+    - `app.kubernetes.io/version`: Determines the version of the `KeptnWorkload` representing the Workload.
+    - `app.kubernetes.io/part-of`: Determines the name of the generated `KeptnApp` representing your
+    Application.
+    All Workloads that share the same value for this label will be consolidated into the same `KeptnApp`.
+
+As an example, consider the following application, consisting of several deployments, which is going to be
+deployed into a KLT-enabled namespace:
+
+```yaml
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: podtato-kubectl
+  annotations:
+    keptn.sh/lifecycle-toolkit: "enabled"
+
+---
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: podtato-head-frontend
+  namespace: podtato-kubectl
+spec:
+  template:
+    metadata:
+      labels:
+        app.kubernetes.io/name: podtato-head-frontend
+        app.kubernetes.io/part-of: podtato-head
+        app.kubernetes.io/version: 0.1.0
+    spec:
+      containers:
+        - name: podtato-head-frontend
+          image: podtato-head-frontend
+---
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: podtato-head-hat
+  namespace: podtato-kubectl
+spec:
+  replicas: 1
+  template:
+    metadata:
+      labels:
+        app.kubernetes.io/name: podtato-head-hat
+        app.kubernetes.io/part-of: podtato-head
+        app.kubernetes.io/version: 0.1.1
+    spec:
+      containers:
+        - name: podtato-head-hat
+          image: podtato-head-hat
+```
+
+Applying these resources will then result in the creation of the following `KeptnApp`:
+
+```yaml
+apiVersion: lifecycle.keptn.sh/v1alpha2
+kind: KeptnApp
+metadata:
+  name: podtato-head
+  namespace: podtato-kubectl
+  annotations:
+    app.kubernetes.io/managed-by: "klt"
+spec:
+  version: "<version string based on a hash of all containing workloads>"
+  workloads:
+  - name: podtato-head-frontend
+    version: 0.1.0
+  - name: podtato-head-hat
+    version: 1.1.1
+```
+
+Due to the creation of this resource, you will now get observability of your application's deployments due to
+the OpenTelemetry tracing features provided by the Keptn Lifecycle Toolkit:
+
+![Application deployment trace](assets/trace.png)