Add public method isFinished() on JaegerSpan

[dougEfresh]

Which problem is this PR solving?

In complicated future/multi-threading environments, such as RX java, you need to pass trace context to the executing threads. In some cases we noticed that spans were activated long after the span has finished (like 30 minutes).
I've noticed hourly repeating tasks used stale trace context. By adding isFinished on the JaegerSpan, we can keep track (via metrics) of spans that are being activated but are themselves finished.

Short description of the changes

Adding public method isFinished to JaegerSpan.

[codecov]

Codecov Report

Merging #634 into master will decrease coverage by 0.23%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #634      +/-   ##
============================================
- Coverage     89.62%   89.38%   -0.24%     
+ Complexity      564      563       -1     
============================================
  Files            69       69              
  Lines          2063     2073      +10     
  Branches        262      263       +1     
============================================
+ Hits           1849     1853       +4     
- Misses          134      138       +4     
- Partials         80       82       +2

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/io/jaegertracing/internal/JaegerSpan.java	93.63% <100%> (+0.11%)	49 <1> (+1)	⬆️
...rtracing/internal/reporters/CompositeReporter.java	71.42% <0%> (-28.58%)	6% <0%> (-1%)
...gertracing/internal/reporters/LoggingReporter.java	81.81% <0%> (-9.1%)	4% <0%> (-1%)
...n/java/io/jaegertracing/internal/JaegerTracer.java	88.92% <0%> (-0.05%)	26% <0%> (ø)

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[yurishkuro]

needs synchronized (this), see finish() method

[dougEfresh]

updated

[yurishkuro]

@dougEfresh are you planning to finish this PR?