Expose current observer from FiberManager

Expose current observer from FiberManager

Summary:
Inteded use case is to write tracing code in common components that can adjust its behavior to the current observer. E.g.
  auto* observer = FiberManager::getUnsafe()->getObserver();
  if (auto* specific_observer = dynamic_cast<SpecificObserver*>(observer)) {
    specific_observer->set_fiber_name(...);
  }

Reviewed By: andriigrynenko

Differential Revision: D3392890

fbshipit-source-id: 284b4964bbb17e9a081779b870af68e8f1a81634