Docs: Add annotations guide for data source plugins (#24432)

* Add annotations guide for data source plugins

* Fix review comments

docs/sources/developers/plugins/add-support-for-annotations.md

++++
+title = "Add support for annotations"
+type = "docs"
++++
+
+# Add support for annotations
+
+This guide explains how to add support for [annotations]({{< relref "../../reference/annotations.md" >}}) to an existing data source plugin.
+
+This guide assumes that you're already familiar with how to [Build a data source plugin]({{< relref "../../../../../tutorials/build-a-data-source-plugin.md" >}}).
+
+Data sources in Grafana can support [Annotations]({{< relref "../../reference/annotations.md" >}}) by handling _annotation queries_.
+
+Handling annotation queries is similar to how you'd handle a metrics query. The difference is that instead of returning [data frames]({{< relref "data-frames.md" >}}), an annotation query returns _annotation events_.
+
+## Add annotations support to your data source
+
+To add logs support to an existing data source, you need to:
+
+- Enable annotations support
+- Override the `annotationQuery` method
+- Construct annotation events
+
+### Enable annotations support
+
+Tell Grafana that your data source plugin can return annotations events by adding `"annotations": true` to the [plugin.json]({{< relref "metadata.md" >}}) file.
+
+```json
+{
+  "annotations": true
+}
+```
+
+### Override the `annotationQuery` method
+
+In `DataSource.ts`, override the `annotationQuery` method from `DataSourceApi`.
+
+```ts
+async annotationQuery(options: AnnotationQueryRequest<MyQuery>): Promise<AnnotationEvent[]> {
+  return [];
+}
+```
+
+### Construct annotation events
+
+Return an array of [AnnotationEvent]({{< relref "../../packages_api/data/annotationevent.md" >}}).
+
+```ts
+async annotationQuery(options: AnnotationQueryRequest<MyQuery>): Promise<AnnotationEvent[]> {
+  const events: AnnotationEvent[] = [];
+
+  const date = new Date();
+
+  const event: AnnotationEvent = {
+    time: date.valueOf(),
+    text: 'foo',
+    tags: ['bar'],
+  };
+
+  events.push(event);
+
+  return events;
+}
+```
+
+## Region annotations
+
+[Region annotations]({{< relref "../../reference/annotations.md#adding-regions-events" >}}) have a start and end time. This can for example be used to annotate maintenance windows or downtime.
+
+To return a region annotation, set the `timeEnd`, and `isRegion` properties.
+
+```ts
+const regionEvent: AnnotationEvent = {
+  time: startDate.valueOf(),
+  timeEnd: endDate.valueOf(),
+  isRegion: true,
+  text: 'foo',
+  tags: ['bar'],
+};
+```
+
+## Build a annotation query editor
+
+Let users write custom annotation queries to only display the annotation events they care about, by adding a _query editor_.
+
+> **Note**: Annotation query editors have yet to receive support for React. The instructions here are given for Angular. Fortunately, you can run Angular even in a plugin otherwise written using React. This section will be updated once React support for annotation queries editors is available.
+
+1. Create a file called `AnnotationQueryEditor.ts` in the plugin root directory, with the following content.
+
+   ```ts
+   export class AnnotationQueryEditor {
+     static templateUrl = 'partials/annotations.editor.html';
+
+     annotation: any;
+
+     constructor() {
+       this.annotation.rawQuery = this.annotation.rawQuery || '';
+     }
+   }
+   ```
+
+1. Create a directory called `partials` in the plugin root directory.
+
+1. Create a file called `annotations.editor.html` in the `partials` directory you just created, with the following content.
+
+   ```html
+   <div class="gf-form-group">
+     <div class="gf-form-inline">
+       <div class="gf-form gf-form--grow">
+         <input
+          class="gf-form-input"
+          placeholder="query expression"
+          ng-model="ctrl.annotation.queryText"
+          ></input>
+       </div>
+     </div>
+   </div>
+   ```
+
+1. In your data source query—the one that extends [DataQuery]({{< relref "../../packages_api/data/dataquery.md" >}})—add the `queryText` property. The name of the property needs to correspond to the text in `ng-model`, e.g. `ctrl.annotation.<propertyName>`.
+
+   ```ts
+   export interface MyQuery extends DataQuery {
+     // ...
+     queryText?: string;
+   }
+   ```
+
+1. In `module.ts`, add the annotation query editor to the plugin.
+
+   ```ts
+   import { AnnotationQueryEditor } from './AnnotationQueryEditor';
+
+   export const plugin = new DataSourcePlugin<DataSource, MyQuery, MyDataSourceOptions>(DataSource)
+     .setConfigEditor(ConfigEditor)
+     .setQueryEditor(QueryEditor)
+     .setAnnotationQueryCtrl(AnnotationQueryEditor);
+   ```
+
+The `queryText` property is now available on the `options` object in the `annotationQuery` method:
+
+```ts
+async annotationQuery(options: AnnotationQueryRequest<MyQuery>): Promise<AnnotationEvent[]> {
+  const expression = options.annotation.queryText;
+
+  // ...
+}
+```

docs/sources/menu.yaml

@@ -360,6 +360,8 @@
           link: /developers/plugins/metadata/
         - name: Data frames
           link: /developers/plugins/data-frames/
+        - name: Add support for annotations
+          link: /developers/plugins/add-support-for-annotations/
         - name: Build a logs data source
           link: /developers/plugins/build-a-logs-data-source/
         - name: Authentication