Add docs showing how to connect to Prometheus UI

- Add docs describing how to connect to th Prometheus UI of a Prometheus
managed app installed on Kubernetes.

- Add a link to this new doc on the development docs index page.

- Add a link to the existing Prometheus metrics docs in the new
Prometheus docs.

- Remove the link to the Prometheus metrics page in the development
docs index page.

- Add link to new doc in Prometheus integrations doc under the
troubleshooting section.

doc/development/README.md

@@ -81,7 +81,7 @@ Complementary reads:
 - [Working with Merge Request diffs](diffs.md)
 - [Kubernetes integration guidelines](kubernetes.md)
 - [Permissions](permissions.md)
-- [Prometheus metrics](prometheus_metrics.md)
+- [Prometheus](prometheus.md)
 - [Guidelines for reusing abstractions](reusing_abstractions.md)
 - [DeclarativePolicy framework](policies.md)
 - [How Git object deduplication works in GitLab](git_object_deduplication.md)

doc/development/prometheus.md

+# Working with Prometheus
+
+For more information on working with [Prometheus metrics](prometheus_metrics.md), see
+the documentation.
+
+## Access the UI of a Prometheus managed application in Kubernetes
+
+You can connect directly to Prometheus, and view the Prometheus user interface, when
+using a Prometheus managed application in Kubernetes:
+
+1. Find the name of the Prometheus pod in the user interface of your Kubernetes
+   provider, such as GKE, or by running the following `kubectl` command in your
+   terminal:
+
+   ```shell
+   kubectl get pods -n gitlab-managed-apps | grep 'prometheus-prometheus-server'
+   ```
+
+   The command should return a result like the following example, where
+   `prometheus-prometheus-server-55b4bd64c9-dpc6b` is the name of the Prometheus pod:
+
+   ```plaintext
+   gitlab-managed-apps  prometheus-prometheus-server-55b4bd64c9-dpc6b  2/2  Running  0  71d
+   ```
+
+1. Run a `kubectl port-forward` command. In the following example, `9090` is the
+   Prometheus server's listening port:
+
+   ```shell
+    kubectl port-forward prometheus-prometheus-server-55b4bd64c9-dpc6b 9090:9090 -n gitlab-managed-apps
+   ```
+
+   The `port-forward` command forwards all requests sent to your system's `9090` port
+   to the `9090` port of the Prometheus pod. If the `9090` port on your system is used
+   by another application, you can change the port number before the colon to your
+   desired port. For example, to forward port `8080` of your local system, change the
+   command to:
+
+   ```shell
+   kubectl port-forward prometheus-prometheus-server-55b4bd64c9-dpc6b 8080:9090 -n gitlab-managed-apps
+   ```
+
+1. Open `localhost:9090` in your browser to display the Prometheus user interface.

doc/user/project/integrations/prometheus.md

@@ -837,6 +837,11 @@ Prerequisites for embedding from a Grafana instance:
 
 ## Troubleshooting
 
+When troubleshooting issues with a managed Prometheus app, it is often useful to
+[view the Prometheus UI](../../../development/prometheus.md#access-the-ui-of-a-prometheus-managed-application-in-kubernetes).
+
+### "No data found" error on Metrics dashboard page
+
 If the "No data found" screen continues to appear, it could be due to:
 
 - No successful deployments have occurred to this environment.