Add documentation for integrated error tracking

Describe core concept and how to enable it.

Signed-off-by: Dmytro Zaporozhets <dzaporozhets@gitlab.com>

doc/operations/error_tracking.md

@@ -108,3 +108,66 @@ clicking the **Resolve** button near the top of the page.
 Marking an error as resolved indicates that the error has stopped firing events. If a GitLab issue is linked to the error, then the issue closes.
 If another event occurs, the error reverts to unresolved.
+
+## Integrated error tracking
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/329596) in GitLab 14.3.
+
+Integrated error tracking is a lightweight alternative to Sentry backend.
+You still use Sentry SDK with your application. But you don't need to deploy Sentry
+or setup for cloud hosted Sentry. Instead you use GitLab as a backend for it.
+
+Sentry backend automatically assigns a Data Source Name (DSN) for every project you create.
+Same with GitLab, you should be able to find a DSN for your project in GitLab error tracking settings.
+By using GitLab provided DSN, your application will connect to GitLab to report an error.
+All those errors will be stored in GitLab database, and then rendered to you in GitLab UI
+the same way like it does for Sentry integration above.
+
+### Feature flag
+
+Integrated error tracking feature is behind a feature flag that is disabled by default.
+
+To enable it:
+
+```ruby
+Feature.enable(:integrated_error_tracking)
+```
+
+To disable it:
+
+```ruby
+Feature.disable(:integrated_error_tracking)
+```
+
+### Project settings
+
+The feature should be enabled on the project level. However, there is no UI to enable this feature yet.
+You need to use GitLab API to enable it.
+
+#### How to enable
+
+Enable `integrated` error tracking setting for your project.
+
+```shell
+curl --request PATCH --header "PRIVATE-TOKEN: <your_access_token>" \
+    "https://gitlab.example.com/api/v4/projects/PROJECT_ID/error_tracking/settings?active=true&integrated=true"
+```
+
+Create a client key (DSN) to use with Sentry SDK in your application. Make sure to save the response as it contains a DSN.
+
+```shell
+curl --request POST --header "PRIVATE-TOKEN: <your_access_token>" \
+    "https://gitlab.example.com/api/v4/projects/PROJECT_ID/error_tracking/client_keys"
+```
+
+Take DSN from the previous step and configure your Sentry SDK with it. New errors will be
+reported to GitLab collector and will be visible in [GitLab UI](#error-tracking-list).
+
+#### How to disable
+
+Same as enable, but with a `false` value instead.
+
+```shell
+curl --request PATCH --header "PRIVATE-TOKEN: <your_access_token>" \
+    "https://gitlab.example.com/api/v4/projects/PROJECT_ID/error_tracking/settings?active=false&integrated=false"
+```