cache auth client

The authClient from the google-auth-library ought to be cached. Otherwise we end up creating a new AuthClient on each request which ends up fetching fresh credentials on every API request. This can potentially double the latency of each API request.

lib/common/util.js

@@ -468,6 +468,8 @@ util.shouldRetryRequest = shouldRetryRequest;
  * for authenticating API requests.
  *
  * @param {object} config - Configuration object.
+ * @param {object=} config.authClient - AuthClient object. If not provided,
+ *     it will be created and cached here.
  * @param {object=} config.credentials - Credentials object.
  * @param {string=} config.email - Account email address, required for PEM/P12
  *     usage.
@@ -476,6 +478,12 @@ util.shouldRetryRequest = shouldRetryRequest;
  * @param {function} callback - The callback function.
  */
 function getAuthClient(config, callback) {
+  if (config.authClient) {
+    setImmediate(function() {
+      callback(null, config.authClient);
+    }
+    return;
+  }
   var googleAuth = new GoogleAuth();
 
   if (config.keyFile) {
@@ -500,6 +508,7 @@ function getAuthClient(config, callback) {
       authClient = authClient.createScoped(config.scopes);
     }
 
+    config.authClient = authClient;
     callback(null, authClient);
   }
 }
@@ -510,6 +519,8 @@ util.getAuthClient = getAuthClient;
  * Authenticate a request by extending its headers object with an access token.
  *
  * @param {object} config - Configuration object.
+ * @param {object=} config.authClient - AuthClient object. If not provided,
+ *     it will be created and cached here.
  * @param {object=} config.credentials - Credentials object.
  * @param {string=} config.email - Account email address, required for PEM/P12
  *     usage.
@@ -563,6 +574,8 @@ util.authorizeRequest = authorizeRequest;
  *     response is related to rate limits or certain intermittent server errors.
  *     We will exponentially backoff subsequent requests by default. (default:
  *     true)
+ * @param {object=} config.authClient - AuthClient object. If not provided,
+ *     it will be created and cached here.
  * @param {object=} config.credentials - Credentials object.
  * @param {boolean=} config.customEndpoint - If true, just return the provided
  *     request options. Default: false.