Create new span if paren span is missing about java-concurrent

[objectiser]

Not sure - I think the role of this library is just to propagate the trace context, not to assume that a span should exist. If the application wants some form of bounding scope/span, then they should ensure it is created through other instrumentations or explicitly by the application itself.

[pavolloffay]

Executors are usually used for time-consuming tasks, therefore having a span which wraps the whole invocation helps you to understand latency problems. Not even saying that In some situations, there won't be parent span (e.g. no servlet environment) in this situation this library does not bring you any value. A specific use case is Executor instrumentation in Spring. It can be used without web package.

Creating a span should be optional. It would be a boolean flag in executor constructor whether to create a span or not.

[malafeev]

fixed via #13
can be closed now