+ Create a diff to recreate object as optional args, since es6 doesn't have real optional args
+ implement special tag handlers
+ add metrics
+ profile performance of Noop tracer vs. Jaeger tracer with const sampling off and on
+ opentracing test harness
+ propagate 1 or many spanRefs froms startSpan options to created span.
+ upgrade span code to use latest jaeger-idl model
+ test span attributes are populated correctly
+ tag values can be any type, so toString is not sufficient
