You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: authentication.rb
    path of file: ./repos/gitlabhq/lib/google_cloud
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

module GoogleCloud
  class Authentication
    def initialize(scope:)
      @scope = scope
    end

    def generate_access_token(client_email, private_key)
      credentials = Google::Auth::ServiceAc
    
    def generate_access_token(client_email, private_key)
      credentials = Google::Auth::ServiceAccountCredentials.make_creds(
        json_key_io: StringIO.new({ client_email: client_email, private_key: private_key }.to_json),
        scope: @scope
      )
      credentials.fetch_access_token!["access_token"]
    rescue StandardError => e
      ::Gitlab::ErrorTracking.track_exception(e, client_email: client_email)
      nil
    end