access to registered tracer

Can we add a method to get access to registered [Tracer](https://github.com/opentracing/opentracing-java/blob/master/opentracing-util/src/main/java/io/opentracing/util/GlobalTracer.java#L67)? We might need access to Tracer for e.g. to create KafkaTracing from Tracer and making this private to GlobalTracer doesn't allow to access it.

