implement dashboard link formatter for GKE

Signed-off-by: Morgan Martinet <morgan.martinet@montreal.ca>

docs/features/kubernetes/configuration.md

@@ -113,9 +113,13 @@ kubectl -n <NAMESPACE> get secret $(kubectl -n <NAMESPACE> get sa <SERVICE_ACCOU
 Specifies the link to the Kubernetes dashboard managing this cluster.
 
 Note that you should specify the app used for the dashboard using the
-**dashboardApp property**, in order to properly format links to kubernetes
+`dashboardApp` property, in order to properly format links to kubernetes
 resources, otherwise it will assume that you're running the standard one.
 
+Note also that this attribute is optional for some kinds of dashboards, such as
+GKE, which requires additional informations specified in the
+`dashboardParameters` option.
+
 ##### `clusters.\*.dashboardApp` (optional)
 
 Specifies the app that provides the Kubernetes dashboard.
@@ -124,11 +128,14 @@ This will be used for formatting links to kubernetes objects inside the
 dashboard.
 
 The supported dashboards are: `standard`, `rancher`, `openshift`, `gke`, `aks`,
-`eks` However, not all of them are implemented yet, so please contribute!
+`eks`. However, not all of them are implemented yet, so please contribute!
 
 Note that it will default to the regular dashboard provided by the Kubernetes
 project (`standard`), that can run in any Kubernetes cluster.
 
+Note that for the `gke` app, you must provide additional information in the
+`dashboardParameters` option.
+
 Note that you can add your own formatter by registering it to the
 `clusterLinksFormatters` dictionary, in the app project.
 
@@ -143,6 +150,43 @@ See also
 https://github.com/backstage/backstage/tree/master/plugins/kubernetes/src/utils/clusterLinks/formatters
 for real examples.
 
+##### `clusters.\*.dashboardParameters` (optional)
+
+Specifies additional information for the selected `dashboardApp` formatter.
+
+Note that, eventhough `dashboardParameters` is optional for some formatters, it
+is mandatory for others, such as GKE.
+
+###### required parameters for GKE
+
+| Name        | Description                                                              |
+| ----------- | ------------------------------------------------------------------------ |
+| projectId   | the ID of the GCP project containing your Kubernetes clusters            |
+| region      | the region of GCP containing your Kubernetes clusters                    |
+| clusterName | the name of your kubernetes cluster, within your `projectId` GCP project |
+
+Note that the GKE cluster locator can automatically provide the values for the
+`dashboardApp` and `dashboardParameters` options if you set the
+`exposeDashboard` property to `true`.
+
+Example:
+
+```yaml
+kubernetes:
+  serviceLocatorMethod:
+    type: 'multiTenant'
+  clusterLocatorMethods:
+    - type: 'config'
+      clusters:
+        - url: http://127.0.0.1:9999
+          name: my-cluster
+          dashboardApp: gke
+          dashboardParameters:
+            projectId: my-project
+            region: us-east1
+            clusterName: my-cluster
+```
+
 ##### `clusters.\*.caData` (optional)
 
 PEM-encoded certificate authority certificates.
@@ -184,6 +228,10 @@ For example:
 Will configure the Kubernetes plugin to connect to all GKE clusters in the
 project `gke-clusters` in the region `europe-west1`.
 
+Note that the GKE cluster locator can automatically provide the values for the
+`dashboardApp` and `dashboardParameters` options if you enable the
+`exposeDashboard` option.
+
 ##### `projectId`
 
 The Google Cloud project to look for Kubernetes clusters in.
@@ -203,6 +251,14 @@ presented by the API server. Defaults to `false`.
 This determines whether the Kubernetes client looks up resource metrics
 CPU/Memory for pods returned by the API server. Defaults to `false`.
 
+##### `exposeDashboard`
+
+This determines wether the `dashboardApp` and `dashboardParameters` should be
+automatically configured in order to expose the GKE dashboard from the
+Kubernetes plugin.
+
+Defaults to `false`.
+
 ### `customResources` (optional)
 
 Configures which [custom resources][3] to look for when returning an entity's