Show proper response structure in vm.getMetadata example (#169)

This updates the documentation for `vm.getMetadata` calls to show the
real response structure returned by calls using `Promise`s.

- [X] Appropriate docs were updated (if necessary)

packages/google-cloud-compute/src/vm.js

@@ -227,8 +227,12 @@ function VM(zone, name) {
      * // If the callback is omitted, we'll return a Promise.
      * //-
      * vm.getMetadata().then(function(data) {
+     *   // Representation of this VM as the API sees it.
      *   const metadata = data[0];
      *   const apiResponse = data[1];
+     *
+     *   // Custom metadata and predefined keys.
+     *   const customMetadata = metadata.metadata;
      * });
      */
     getMetadata: true,