function callbacks(err, creds) {
      var cb;
      while ((cb = self.loadCredentialsCallbacks.shift()) !== undefined) {
        cb(err, creds);
      }
    }
Loads a set of credentials stored in the instance metadata service

@api private
@callback callback function(err, credentials)
  Called when credentials are loaded from the resource
  @param err [Error] if an error occurred, this value will be set
  @param credentials [Object] the raw JSON object containing all
    metadata from the credentials resource