CARBON-15115: Initial version of org.wso2.carbon.metrics API.

There are two components. The org.wso2.carbon.metrics.manager component has the public API for WSO2 Metrics and it is exposed via org.wso2.carbon.metrics.manager.MetricManager class. The org.wso2.carbon.metrics.impl component has the implementation, which uses Metrics library (http://dropwizard.github.io/metrics/).