Give reporters a final chance to flush data before stopping

[ivantopo]

In the particular case of all reporters being shut down because of the JVM exiting we should have a means of reporting whatever data has been accumulated since the last tick before completely stopping the reporters. I think the flow should be:

1. Kamon.stopAllReporters() is called
2. Whatever state is available for metrics and span reporters is flushed via reportPeriodSnapshots/reportSpans
3. The stop hook is called
4. After completing all the above successfully or a timeout is reached Kamon finishes shutting down the reporters.

[l15k4]

Yeah this is pretty important. I run a lot of kubernetes jobs that just shut jvm down after their are done and metrics are not flushed after Kamon.stopAllReporters() ... I would simply have to Thread.sleep :-/

Also some monitoring services like Stackdriver doesn't allow for tick-interval less than 15 seconds as you'd reach quota... so I'd have to Thread.sleep(15000) :-/