Add missing API docs for Datadog integration

doc/api/integrations.md

@@ -300,6 +300,44 @@ Get Campfire integration settings for a project.
 GET /projects/:id/integrations/campfire
 ```
 
+## Datadog
+
+Datadog system monitoring.
+
+### Create/Edit Datadog service
+
+Set Datadog service for a project.
+
+```plaintext
+PUT /projects/:id/services/datadog
+```
+
+Parameters:
+
+| Parameter | Type | Required | Description |
+| --------- | ---- | -------- | ----------- |
+| `api_key`         | string | true  | API key used for authentication with Datadog |
+| `api_url`         | string | false | (Advanced) Define the full URL for your Datadog site directly |
+| `datadog_site`    | string | false | Choose the Datadog site to send data to. Set to `datadoghq.eu` to send data to the EU site |
+| `datadog_service` | string | false | Name of this GitLab instance that all data will be tagged with |
+| `datadog_env`     | string | false | The environment tag that traces will be tagged with |
+
+### Delete Datadog service
+
+Delete Datadog service for a project.
+
+```plaintext
+DELETE /projects/:id/services/datadog
+```
+
+### Get Datadog service settings
+
+Get Datadog service settings for a project.
+
+```plaintext
+GET /projects/:id/services/datadog
+```
+
 ## Unify Circuit
 
 Unify Circuit RTC and collaboration tool.