feat: Add EventType auto create to experimental features (#5631)

* feat: Add EventType auto create to experimental features

* Add new page to navigation config

* Decrease title size

* Split broker create to kn/yaml variants

* Polish event sending section

* Align opt-in formatting

* Correct feature name

config/nav.yml

@@ -279,6 +279,7 @@ nav:
         - New Trigger Filters: eventing/experimental-features/new-trigger-filters.md
         - KReference.Group field: eventing/experimental-features/kreference-group.md
         - Knative reference mapping: eventing/experimental-features/kreference-mapping.md
+        - EventType auto creation: eventing/experimental-features/eventtype-auto-creation.md
     # Eventing reference docs
       - Reference:
         - Eventing API: eventing/reference/eventing-api.md

docs/eventing/experimental-features/eventtype-auto-creation.md

@@ -0,0 +1,114 @@
+# EventType auto creation for Brokers
+
+**Flag name**: `eventtype-auto-creation`
+
+**Stage**: Alpha, disabled by default
+
+**Tracking issue**: [#7044](https://github.com/knative/eventing/issues/7044)
+
+**Persona**: Developer
+
+
+## Overview
+
+With the `eventtype-auto-creation` feature, we have possibliy to _auto create_ EventTypes that are received and ingressed by the Knative Broker implementations.
+
+For making use of this _opt-in_ feature, we must turn it on in the `config-features`, by setting the `eventtype-auto-creation` flag to `enabled`:
+
+```yaml
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: config-features
+  namespace: knative-eventing
+data:
+  eventtype-auto-creation: "enabled"
+...
+```
+
+With this experiemental feature enabled, we get `EventType`s on the broker ingress for free. Instead of manually creating them as yaml manifests along the application code that talks to the `Broker` API. 
+
+## Example
+
+To check the feature is working, create a simple broker:
+
+
+=== "kn CLI"
+
+    ```bash
+    kn broker create my-broker
+    ```
+
+=== "Apply YAML"
+
+    1. Create a YAML file using the following example:
+
+        ```yaml
+        apiVersion: eventing.knative.dev/v1
+        kind: Broker
+        metadata:
+          namespace: default
+          name: my-broker
+        ```
+
+    1. Apply the YAML file by running the command:
+
+          ```bash
+          kubectl apply -f <filename>.yaml
+          ```
+          Where `<filename>` is the name of the file you created in the previous step.
+
+
+## Produce Events to the Broker
+
+The auto-creation feature is triggered by processed events. Therefore to verify the functionality we need to send a sample event with desired type. This can be achieved in a severals ways, below are two examples using `kn-plugin-event` and `cURL`
+container in a cluster.
+
+=== "kn event"
+
+    Below is an example that sends an event with the `kn` CLI, for improved developer productivity. The `kn-plugin-event` plugin can be installed with Homebrew or downloaded directly from GitHub releases, for more details please refer to plugin [instalation steps](https://knative.dev/docs/client/kn-plugins/).
+
+    1. Setup `kn event` plugin
+        ```bash
+        brew install knative-sandbox/kn-plugins/event
+        ```
+
+    1. Send event
+        ```bash
+        kn event send \
+          --to Broker:eventing.knative.dev/v1:my-broker\
+          --type com.corp.integration.warning \
+          -f message="There might be a problem"
+        ```
+
+=== "cURL container"
+
+    An event can be send via `curl` in a cluster:
+
+    ```bash
+    kubectl run curl  --image=docker.io/curlimages/curl --rm=true --restart=Never -ti \
+      -- -X POST -v \
+      -H "content-type: application/json" \
+      -H "ce-specversion: 1.0" \
+      -H "ce-source: my/curl/command" \
+      -H "ce-type: my.demo.event" \
+      -H "ce-id: 6cf17c7b-30b1-45a6-80b0-4cf58c92b947" \
+      -d '{"name":"Knative Demo"}' \
+      http://broker-ingress.knative-eventing.svc.cluster.local/default/my-broker
+    ```
+    This is more complex, as we have to _craft_ the event as part of the `curl` HTTP POST request.
+
+## Event Discovery
+
+After the two produced events, we should be able to have discoverable events in the system, based on the `eventtype-auto-creation` feature:
+
+```
+k get eventtypes.eventing.knative.dev -A 
+NAMESPACE   NAME     TYPE                           SOURCE            SCHEMA   BROKER      DESCRIPTION   READY   REASON
+default     <...>    com.corp.integration.warning   kn-event/v1.9.0            my-broker                 True    
+default     <...>    my.demo.event                  my/curl/command            my-broker                 True    
+```
+
+## Conclusion and Recommendation
+
+With out this feature we would not see the two `EventType` instances in the system, so we have improved the discoverablilty of events, for consumption. However while this _opt-in_ feature is handy for automatic event creation, we **strongly** recommend to create the actual `EventType` manifests for all events that your application `deployments` produce, as part of your Gitops pipeline, rather than relying on this auto-create feature.