Unify Metrics and Events

Currently, the Monasca API supports metrics only. Events will be added with the target being StackTach capabilities. Need to define a new metric/event resource that will handle both metrics and events. The specification of a metric/event will include the ability to specific intrinsic and extrinsic meta data. Intrinsic meta data is used to uniquely identify the metric similar to dimensions. Extrinsic meta data is used for supplying additional information, but is not used to uniquely identify the metric/event.