// IMPORTANT: don't cache a reference to `other` or `other.Handler` here.
        // We need to allow it to be GC'ed.
        public ExpiredHandlerTrackingEntry(ActiveHandlerTrackingEntry other)
        {
            Key = other.Key;

            _livenessTracker = new WeakReference(other.Handler);
            InnerHandler     = other.Handler.InnerHandler;
        }
 protected virtual void StartHandlerEntryTimer(ActiveHandlerTrackingEntry entry) => entry.StartExpiryTimer(_expiryCallback);