out_stackdriver: add k8s_cluster resource type

pipeline/outputs/stackdriver.md

@@ -22,7 +22,7 @@ Before to get started with the plugin configuration, make sure to obtain the pro
 | job | An identifier for a grouping of related task, such as the name of a microservice or distributed batch. If the resource type is _generic\_task_, then this field is required. |  |
 | task\_id | A unique identifier for the task within the namespace and job, such as a replica index identifying the task within the job. If the resource type is _generic\_task_, then this field is required. |  |
 | export\_to\_project\_id | The GCP project that should receive these logs. | Defaults to the project ID of the google\_service\_credentials file, or the project\_id from Google's metadata.google.internal server. |
-| resource | Set resource type of data. Supported resource types: _k8s\_container_, _k8s\_node_, _k8s\_pod_, _global_, _generic\_node_, _generic\_task_, and _gce\_instance_. | global, gce\_instance |
+| resource | Set resource type of data. Supported resource types: _k8s\_container_, _k8s\_node_, _k8s\_pod_, _k8s\_cluster_, _global_, _generic\_node_, _generic\_task_, and _gce\_instance_. | global, gce\_instance |
 | k8s\_cluster\_name | The name of the cluster that the container \(node or pod based on the resource type\) is running in. If the resource type is one of the _k8s\_container_, _k8s\_node_ or _k8s\_pod_, then this field is required. |  |
 | k8s\_cluster\_location | The physical location of the cluster that contains \(node or pod based on the resource type\) the container. If the resource type is one of the _k8s\_container_, _k8s\_node_ or _k8s\_pod_, then this field is required. |  |
 | labels\_key | The value of this field is used by the Stackdriver output plugin to find the related labels from jsonPayload and then extract the value of it to set the LogEntry Labels. | logging.googleapis.com/labels |