Update licence header in MetricAPI
/*
- * Copyright (c) 2020, WSO2 Inc. (http://www.wso2.org) All Rights Reserved.
+ * Copyright (c) 2019, WSO2 Inc. (http://www.wso2.org) All Rights Reserved.
*
* WSO2 Inc. licenses this file to you under the Apache License,
* Version 2.0 (the "License"); you may not use this file except
