#11 Add TracingSubscriber for RxJava2 Flowable usecases

[rossdanderson]

I've added TracingSubscriber (implementing FlowableSubscriber) which covers use cases where you need support for managing back-pressure.
As part of this I have renamed AbstractTracingObserver to RxTracer and made it a field of the implementations rather than inheriting it, which means we can use the same code for the TracingSubscriber as well as the TracingObserver - I've checked that this does not affect the public API as this was already package private.

I have had to make a change to the design for TracingSubscriber in how it is created, by adding static create methods for the various possibilities matching Flowable.subscribe(...). This is mostly because I don't really understand the purpose of TracingConsumer and I wouldn't know what to call a Subscriber equivalent to this (they also use Consumers underneath so it could get confusing!).

An example of wrapping an existing subscriber currently looks like:

        Flowable<Integer> flowable = createSequentialFlowable(mockTracer);
        TestSubscriber<Integer> subscriber = new TestSubscriber<>();
        flowable.subscribe(TracingSubscriber.create(subscriber, "sequential", mockTracer));

and an example of creating a TracingSubscriber directly:

        Flowable<Integer> flowable = createSequentialFlowable(mockTracer).subscribe(
                TracingSubscriber.create(o -> { System.out.println(o); }, "sequential", mockTracer)
        );

Unless I am missing the reason for it's existence, I would consider deprecating/removing TracingConsumer and replacing it with static create methods on TracingObserver.

If you have suggestions, please let me know!

[coveralls]

Pull Request Test Coverage Report for Build 94

• 24 of 41 (58.54%) changed or added relevant lines in 4 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.7%) to 62.824%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
opentracing-rxjava-2/src/main/java/io/opentracing/rxjava2/TracingConsumer.java	3	4	75.0%
opentracing-rxjava-2/src/main/java/io/opentracing/rxjava2/TracingObserver.java	3	4	75.0%
opentracing-rxjava-2/src/main/java/io/opentracing/rxjava2/TracingSubscriber.java	16	31	51.61%

Files with Coverage Reduction	New Missed Lines	%
opentracing-rxjava-2/src/main/java/io/opentracing/rxjava2/TracingConsumer.java	1	73.53%

Totals
Change from base Build 93:	-0.7%
Covered Lines:	218
Relevant Lines:	347

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💛 - Coveralls