add a convenience function for span creation

[ivantopo]

This PR brings a simple function to create a Span to wrap the execution of code blocks, with two special behaviors:

• The function stores the created Span in the current context while the function is executed.
• The function will always finish the Span after the wrapped code finishes execution, with special handling for Scala Futures and CompletionStage instances. If the wrapped code returns a Future or CompletionStage, the created Span will be finished when the returned Future/CompletionStage completes.

This function should be enough to replace the soon-to-be-deprecated functions in ScalaFutureInstrumentation. More info here: #1021.

Also, this simplifies cases where people want a simple Span for a block of code and end up having to write something like this:

Kamon.runWithSpan(Kamon.spanBuilder(....).start()) {

}

[SimunKaracic]

LGTM, merge when ready

[AlexITC]

Shouldn't .fail(t) be invoked before finishing the span?

[the-overengineer]

It's being handled by runWithSpan for this case, so no worries :)

Kamon/core/kamon-core/src/main/scala/kamon/ContextStorage.scala

Line 119 in ce27fb5

@inline def runWithSpan[T](span: Span, finishSpan: Boolean)(f: => T): T = {