Fix example prometheus.yml to use "external_labels" instead of "labels".

documentation/examples/prometheus.yml

@@ -4,8 +4,9 @@ global:
   evaluation_interval: 15s # By default, scrape targets every 15 seconds.
   # scrape_timeout is set to the global default (10s).
 
-  # Attach these extra labels to all timeseries collected by this Prometheus instance.
-  labels:
+  # Attach these labels to any time series or alerts when communicating with
+  # external systems (federation, remote storage, Alertmanager).
+  external_labels:
       monitor: 'codelab-monitor'
 
 # Load and evaluate rules in this file every 'evaluation_interval' seconds.
@@ -27,4 +28,4 @@ scrape_configs:
     # scheme defaults to 'http'.
 
     target_groups:
-      - targets: ['localhost:9090']
+      - targets: ['localhost:9090']