Document the new __meta_gce_instance_id discovery label.
@@ -589,6 +589,7 @@ address with relabeling.
The following meta labels are available on targets during [relabeling](#relabel_config):
+* `__meta_gce_instance_id`: the numeric id of the instance
* `__meta_gce_instance_name`: the name of the instance
* `__meta_gce_label_<name>`: each GCE label of the instance
* `__meta_gce_machine_type`: full or partial URL of the machine type of the instance
