Bug: Unsubscribing from DiagnosticSource events does not work

[gregkalapos]

This is more pressuring for writing tests.

Currently code like this does not unsubscribe the agent from receiving events and creating spans:

using (agent.Subscribe(new HttpDiagnosticsSubscriber()))
{
	//do stuff			
}
//we should not be subscribed here, but we are.

Therefore subsequent tests that assume the HttpDiagnosticsSubscriber is not registered will fail.

[gregkalapos]

The root cause was in DiagnosticInitializer.cs:

public void OnNext(DiagnosticListener value)
{
   foreach (var listener in _listeners)
   {
      if (value.Name == listener.Name) value.Subscribe(listener);
   }
}

value.Subscribe returns IDisposable which wasn't disposed. Fix is on the way.

[gregkalapos]

Fixed by #75