Keep count of outstanding open spans

[black-adder]

We have numerous cases where users ask us why they can't see spans being persisted. One big cause is the user closes the tracer before the spans are persisted or they forget to finish the span.

If we keep a simple counter of number of started spans (and decrement per finish), then on tracer.close(), we can emit a log saying that: "N spans were started but not finished()".

Should be relatively easy to implement and will assist in debugging issues (unless no logger is configured...)

[yurishkuro]

if we do that, it's not that far from actually preemptively flushing the spans that are potentially left unfinished. If we add a flag "in-progress" to the span, and change the merge function on the backend to recognize it, we could be able to support partial spans.

[de-robat]

for reference, supporting "partial spans" is an ongoing discussion at the jaeger-client over here jaegertracing/jaeger-client-java#231

[yurishkuro]

Per #3362, we're sunsetting Jaeger clients.