Ability to create not started span or manual assign parent span

[mharj]

Is your feature request related to a problem? Please describe.

I'm implmenting spans to track progress and I think when callback is triggered from outside of scope we loose span hierarcy.
My current hack is to create tracer.startSpan early, but then I can't really see actual call runtime.

Describe the solution you'd like

const abortSpan = tracer.createSpan('abort-callback');

const onAbortCallback = () => {
  abortSpan.start();
  ...
  abortSpan.end();
}
signal.addEventListener('abort', onAbortCallback );

Or alteratively, ability to make manual link to parent Span when starting new span.

const onAbortCallback = () => {
  return tracer.startActiveSpan('abort-callback', {parent: parenSpan}),  (abortSpan) => {
    abortSpan.start();
    ...
    abortSpan.end();
  });
}
signal.addEventListener('abort', onAbortCallback );

Or create some sort ref like SpanRef which can be still ref.startActiveSpan() to keep span ref in scope.

Describe alternatives you've considered

Create tracer.startSpan early, but this loses actualy time used on callback.
Tested also binding callback in many different ways, but this just looses tracer hierarcy if not created before actual call.