Add docs page for OOTB metrics dashboards

Add a new docs page describing the OOTB metrics dashboards that
GitLab provides.

doc/operations/metrics/dashboards/default.md

+---
+stage: Monitor
+group: APM
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
+---
+
+# GitLab-defined metrics dashboards **(CORE)**
+
+GitLab provides some dashboards out-of-the-box for any project with
+[Prometheus available](../../../user/project/integrations/prometheus.md). You can
+[duplicate these GitLab-defined dashboards](index.md#duplicate-a-gitlab-defined-dashboard):
+
+- [Overview dashboard](#overview-dashboard).
+- [Kubernetes pod health dashboard](#kubernetes-pod-health-dashboard).
+
+To learn about the components of a dashboard, read
+[Metrics dashboard for your CI/CD environment](../index.md).
+
+## Overview dashboard
+
+This dashboard is the default metrics dashboard. It displays a large number of
+metrics about the [deployed application](../index.md#configure-prometheus-to-gather-metrics).
+
+![Example of metrics dashboard](../img/example-dashboard_v13_3.png)
+
+## Kubernetes pod health dashboard
+
+NOTE: **Note:**
+This dashboard requires Kubernetes v1.14 or higher, due to the
+[change in metric labels](https://github.com/kubernetes/kubernetes/pull/69099)
+in Kubernetes 1.14.
+
+This dashboard displays CPU, memory, network and disk metrics for the pods in your
+[connected K8s cluster](../../../user/project/clusters/index.md). It provides a
+[variable selector](templating_variables.md#metric_label_values-variable-type)
+at the top of the dashboard to select which pod's metrics to display.

doc/operations/metrics/dashboards/index.md

@@ -8,7 +8,7 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/59974) in GitLab 12.1.
 
-By default, all projects include a GitLab-defined Prometheus dashboard, which
+By default, all projects include a [GitLab-defined Prometheus dashboard](default.md), which
 includes a few key metrics, but you can also define your own custom dashboards.
 
 You may create a [new dashboard from scratch](#add-a-new-dashboard-to-your-project)
@@ -23,7 +23,8 @@ The metrics as defined below do not support alerts, unlike
 > UI option [introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/228856) in GitLab 13.3.
 
 You can configure a custom dashboard by adding a new YAML file into your project's
-`.gitlab/dashboards/` directory. For the dashboard to display on your project's **Operations > Metrics** page, the files must have a `.yml`
+`.gitlab/dashboards/` directory. For the dashboard to display on your project's
+**Operations > Metrics** page, the files must have a `.yml`
 extension and be present in your project's **default** branch.
 
 To create a new dashboard from the GitLab user interface:
@@ -145,7 +146,7 @@ Your custom dashboard is available at `https://example.com/project/-/metrics/cus
 
 To manage the settings for your metrics dashboard:
 
-1. Sign in as a user with project Maintainer or Admin
+1. Sign in as a user with project Maintainer or Administrator
    [permissions](../../../user/permissions.md#project-members-permissions).
 1. Navigate to your dashboard at **Operations > Metrics**.
 1. In the top-right corner of your dashboard, click **Metrics Settings**:

doc/operations/metrics/index.md

@@ -72,16 +72,15 @@ and NGINX, and attempts to identify individual environments. To learn more about
 the supported metrics and scan processes, see the
 [Prometheus Metrics Library documentation](../../user/project/integrations/prometheus_library/index.md).
 
-To view the metrics dashboard for an environment that has
-To view the metrics dashboard for an environment that is
+To view the [default metrics dashboard](dashboards/default.md) for an environment that is
 [configured to gather metrics](#configure-prometheus-to-gather-metrics):
 
 1. *If the metrics dashboard is only visible to project members,* sign in to
    GitLab as a member of a project. Learn more about [metrics dashboard visibility](#metrics-dashboard-visibility).
 1. In your project, navigate to **Operations > Metrics**.
 
-GitLab displays the default metrics dashboard for the environment, like the
-following example:
+GitLab displays the [default metrics dashboard](dashboards/default.md) for the environment,
+like the following example:
 
 ![Example of metrics dashboard](img/example-dashboard_v13_3.png)