Implement Controller for metric

[cijothomas]

Parent: #316

From the spec: "A controller is needed to coordinate the decision to begin collection. Controllers generally are responsible for binding the Meter implementation, the Batcher, and the Exporter."

There are no controllers yet in current implementation. There should be atleast one Controller implementation provided as part of SDK which can trigger collection at fixed regular intervals. This should also take care of 'flushing' any metrics on a need basis (customer initiated) or when application is shutting down (automatically)