Updated copyright

Added OpenFaaS Author(s) to show distributed copyright.

Signed-off-by: Ken Fukuyama <kenfdev@gmail.com>

gateway/metrics/exporter.go

-// Copyright (c) Alex Ellis 2017. All rights reserved.
+// Copyright (c) Alex Ellis 2017
+// Copyright (c) 2018 OpenFaaS Author(s)
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
 package metrics