[mdatagen] Add docs for optional events (#13131)

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Ex. Adding a feature - Explain what this achieves.-->
#### Description
Add documentation template for optional events.

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#### Link to tracking issue
Part of #12571 

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#### Testing
n/a

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#### Documentation
Added

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Author: sincejune

cmd/mdatagen/internal/samplereceiver/documentation.md

@@ -141,6 +141,30 @@ The event will be removed soon.
 | slice_attr | Attribute with a slice value. | Any Slice |
 | map_attr | Attribute with a map value. | Any Map |
 
+## Optional Events
+
+The following events are not emitted by default. Each of them can be enabled by applying the following configuration:
+
+```yaml
+events:
+  <event_name>:
+    enabled: true
+```
+
+### default.event.to_be_renamed
+
+[DEPRECATED] Example event disabled by default.
+
+The event will be renamed soon.
+
+#### Attributes
+
+| Name | Description | Values |
+| ---- | ----------- | ------ |
+| string_attr | Attribute with any string value. | Any Str |
+| boolean_attr | Attribute with a boolean value. | Any Bool |
+| boolean_attr2 | Another attribute with a boolean value. | Any Bool |
+
 ## Resource Attributes
 
 | Name | Description | Values | Enabled |

cmd/mdatagen/internal/templates/documentation.md.tmpl

@@ -174,6 +174,29 @@ events:
 {{- end }}
 {{- end }}
 
+{{- $optionalEventSeen := false }}
+{{- range $eventName, $event := .Events }}
+{{- if not $event.Enabled }}
+{{- if not $optionalEventSeen }}
+
+## Optional Events
+
+The following events are not emitted by default. Each of them can be enabled by applying the following configuration:
+
+```yaml
+events:
+  <event_name>:
+    enabled: true
+```
+
+{{- end }}
+{{- $optionalEventSeen = true }}
+
+{{ template "event-documentation" $eventName }}
+
+{{- end }}
+{{- end }}
+
 {{- if .ResourceAttributes }}
 
 ## Resource Attributes