Add an example for additional_label option.

examples/additional_label/cloudprober.cfg

@@ -0,0 +1,28 @@
+# You can add additional labels to the probe results using the probe field,
+# "additional_label".  An additional label's value can be static or it can be
+# derived from the target labels.
+#
+# For the following config, probe results will look like the following:
+# total{probe=google_com,ptype=http,src_zone=us-east1-c,env=prod}: 90
+# success{probe=google_com,ptype=http,src_zone=us-east1-c,env=prod}: 80
+probe {
+    name: "google_com"
+    type: HTTP
+    targets {
+        host_names: "www.google.com"
+    }
+    interval_msec: 5000
+    timeout_msec: 1000
+
+    additional_label {
+      key: "src_zone"
+      value: "{{.zone}}"    # It will be replaced by GCE zone if running on GCE.  
+    } 
+
+    additional_label { 
+      key: "env"
+      value: "prod"
+    }
+
+    http_probe {}
+}