Remove semaphore acquire from span

[endorama]

To compute more precise timing on the execution of HandleStream and allow monitoring semaphore operation using other instrumentation, move the span start in HandleStream after semaphore acquisition.

This prevents the context to be rewritten when passed to sem.Acquire.

[kruskall]

I'd prefer to create a span inside semAcquire in apm-data and avoid passing a custom semaphore to enable tracing. This way support would be transparent just like we have tracing for the Stream method.
I'm also concerned about the above PR effect on APM server: we're not passing a custom semaphore there so that would create misleading traces (Stream would look really fast and any amount of time spent acquiring the lock would be lost)

[endorama]

Closing this PR in favor of #190