Update fetchMetadataToken (#4240)

Author: matthewzhaocc

.changes/next-release/feature-MetadataService-3a550e8f.json

@@ -0,0 +1,5 @@
+{
+  "type": "feature",
+  "category": "MetadataService",
+  "description": "Update AWS.MetadataService to expose the fetchMetadataToken function."
+}

lib/metadata_service.d.ts

@@ -17,6 +17,10 @@ export class MetadataService {
         options: {method?: string, headers?: {[key: string]: String} },
         callback: (err: AWSError, data: string) => void
     ): void;
+    /**
+     * Fetches metadata token used for authenticating against the instance metadata service.
+     */
+    fetchMetadataToken(callback: (err: AWSError, token: string) => void): void;
     /**
      * 169.254.169.254
      */

lib/metadata_service.js

@@ -120,9 +120,8 @@ AWS.MetadataService = inherit({
   loadCredentialsCallbacks: [],
 
   /**
-   * Fetches metadata token used for getting credentials
+   * Fetches metadata token used for authenticating against the instance metadata service.
    *
-   * @api private
    * @callback callback function(err, token)
    *   Called when token is loaded from the resource
    */