Allow a user to add a JVM shutdown hook to drain metrics queue #1296

Metrics published asynchronously are sometimes lost when the JVM shuts down, which can happen relatively often in serverless environments. This usually is not a problem, because there is usually little impact from the loss of a few metrics, but in some cases the user would want to make sure the metrics are fully released before the JVM shutdown finishes. A shutdown hook (or similar mechanism) should be introduced to allow this.