jaeger-client-java does not automatically flush when program exits

[ackerleytng]

Requirement

I was trying to get the opentracing tutorial to work with the latest version of jaeger-client-java and it wasn't flushing the span to the jaeger server

Problem

The span is not getting flushed to the jaeger server

Please see line 36/37 of
https://github.com/ackerleytng/opentracing-tutorial-debug/blob/master/java/src/main/java/lesson01/exercise/Hello.java

Proposal

@jpkrohling suggested registering the tracer with a shutdown hook, to flush it when the JVM goes down