* Fix tracing spans are not being forwarded to spawned task

There is a bug that tracing spans are not forwarded to spawned task. The
problem was that only the telemetry span was forwarded. The solution to
this is to use the tracing provided `in_current_span` to capture the
current active span and pass the telemetry span explictely. We will now
always enter the span when the future is polled. This is essentially the
same strategy as tracing is doing with its `Instrumented`, but now
extended for our use case with having multiple spans active.

* More tests