Allow access to a FiberManager's currently running fiber

Summary: Allow access to a FiberManager's currently running fiber

Reviewed By: @alikhtarov, @andriigrynenko

Differential Revision: D2257201

folly/experimental/fibers/FiberManager.h

@@ -224,6 +224,13 @@ class FiberManager : public ::folly::Executor {
    */
   bool hasActiveFiber() const;
 
+  /**
+   * @return The currently running fiber or null if no fiber is executing.
+   */
+  Fiber* currentFiber() const {
+    return currentFiber_;
+  }
+
   /**
    * @return What was the most observed fiber stack usage (in bytes).
    */