Short lived applications run the risk of losing metrics that are
generated near the end of their lifetimes because MetricSinks can
and do buffer data locally. The exact amount of data loss depends on
MetricSink implementation, caller configuration, and the timing of
the last metric sync.

This adds a Shutdown function to the package API and the MetricSink
interface. Shutdown flushes locally buffered data to storage, and
then frees resources allocated to the MetricSink.

Currently not all MetricSinks support exiting, and so resource
release when calling Shutdown is best effort. Since Shutdown is
intended for use immediately prior to application exit this is deemed
acceptable, since resource leakage is minimized in this case.