Update stackdriver.md on clarifying the use of log_name_key and labels_key (#637)

* Update stackdriver.md

Signed-off-by: Sophie Fang <sophieyfang@google.com>

* Update stackdriver.md

Signed-off-by: Sophie Fang <sophieyfang@google.com>

* Update stackdriver.md

Signed-off-by: Sophie Fang <sophieyfang@google.com>

Author: sophieyfang

pipeline/outputs/stackdriver.md

@@ -25,7 +25,8 @@ Before to get started with the plugin configuration, make sure to obtain the pro
 | 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                                                                                                                |
 | 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                                                                                                       |
+| 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](https://cloud.google.com/logging/docs/reference/v2/rest/v2/LogEntry) `labels` field.                                                                                                                     | logging.googleapis.com/labels                                                                                                        |
+| log_name_key               | The value of this field is used by the Stackdriver output plugin to find the related logName from jsonPayload and then extract the value of it to set the [LogEntry](https://cloud.google.com/logging/docs/reference/v2/rest/v2/LogEntry) `logName` field.                                                                                                                     | logging.googleapis.com/logName                                                                                                      |
 | tag_prefix                 | Set the tag_prefix used to validate the tag of logs with k8s resource type. Without this option, the tag of the log must be in format of k8s_container(pod/node).\* in order to use the k8s_container resource type. Now the tag prefix is configurable by this option (note the ending dot). | k8s_container., k8s_pod., k8s_node.                                                                                                 |
 | severity_key               | Specify the name of the key from the original record that contains the severity information.                                                                                                                                                                                                  |                                                                                                                                     |
 | tag_prefix                 | Set the tag_prefix used to validate the tag of logs with k8s resource type. Without this option, the tag of the log must be in format of k8s_container(pod/node).\* in order to use the k8s_container resource type. Now the tag prefix is configurable by this option.                       | k8s_container., k8s_pod., k8s_node.                                                                                                 |