Docs: Add 12-factor how-to sections for integrations

docs/howto/manage-web-app-charms/integrate-web-app-charm.rst

@@ -32,7 +32,14 @@ After the integration has been established, the connection string and other
 configuration options will be available as environment variables that you may
 use to configure your 12-factor application.
 
-For example, if you wish to integrate your 12-factor application with PostgreSQL
+.. seealso::
+
+  :external+ops:doc:`Ops | How to manage relations <howto/manage-relations>`
+
+Integrate with a database
+-------------------------
+
+If you wish to integrate your 12-factor web app with PostgreSQL
 (`machine <https://charmhub.io/postgresql>`_ or
 `k8s <https://charmhub.io/postgresql-k8s>`_
 charm), add the following endpoint definition to your project file:
@@ -65,4 +72,45 @@ configure your 12-factor application.
 - ``POSTGRESQL_DB_HOSTNAME``
 - ``POSTGRESQL_DB_PORT``
 
-    See also: :external+ops:doc:`Ops | How to manage relations <howto/manage-relations>`
+Integrate with ingress
+----------------------
+
+If you wish to integrate your 12-factor web app with an ingress,
+for instance
+`Nginx Ingress Integrator <https://charmhub.io/nginx-ingress-integrator>`_,
+provide the integration to your deployed app with:
+
+.. code-block:: bash
+
+    juju integrate <app charm> nginx-ingress-integrator
+
+You don't need to add an endpoint definition to your charm's
+project file.
+
+Integrate with observability
+----------------------------
+
+You must prepare an ingress if you wish to integrate your 12-factor web app
+with the `Canonical Observability Stack
+(COS) <https://charmhub.io/topics/canonical-observability-stack>`_.
+COS relies on the Traefik ingress to expose, for example, Grafana.
+On MicroK8s, Traefik requires the MetalLB loadbalancer to be enabled which
+requires an IP range. Provide the IP range and enable the addon with:
+
+.. code-block:: bash
+
+    IPADDR=$(ip -4 -j route get 2.2.2.2 | jq -r '.[] | .prefsrc')
+    microk8s enable metallb:$IPADDR-$IPADDR
+
+
+Deploy and integrate observability to the 12-factor app with:
+
+.. code-block:: bash
+
+    juju deploy cos-lite --trust
+    juju integrate <app charm> grafana
+    juju integrate <app charm> prometheus
+    juju integrate <app charm> loki
+
+You don't need to add endpoint definitions to your charm's
+project file.