Expose timepoint at the start of EventBase loop callbacks

Summary: This is useful to separate out idle time from the evb loop time.

Reviewed By: yfeldblum

Differential Revision: D46956487

fbshipit-source-id: fbf99b15b8b8c22df65f77b9624543d7b0a999bf

folly/io/async/EventBase.h

@@ -869,6 +869,13 @@ class EventBase : public TimeoutManager,
    */
   std::thread::id getLoopThreadId();
 
+  /**
+   * Returns the timepoint at the start of the loop callbacks.
+   */
+  std::chrono::steady_clock::time_point getLoopCallbacksStartTime() {
+    return startWork_;
+  }
+
   /// Implements the Executor interface
   void add(Cob fn) override { runInEventBaseThread(std::move(fn)); }