Updates supported Go versions to 1.16, 1.17 and 1.18.

Adds a Shutdown function that attempts to insure buffered metrics are flushed to the underlying sink for short-lived processes. See https://github.com/armon/go-metrics/pull/132 and https://github.com/armon/go-metrics/pull/134.

Note that Shutdown was initially added and released as a 0.3.11 but contained a breaking change for library users that have their own MetricsSink implementation since it added an interface method.

This release retracts the 0.3.11 release and re-introduces the Shutdown change in a backwards compatible manner. See https://github.com/armon/go-metrics/pull/134.