update enum

Author: blink1073

pymongo/monitoring.py

@@ -934,6 +934,9 @@ class ConnectionClosedReason:
     POOL_CLOSED = "poolClosed"
     """The pool was closed, making the connection no longer valid."""
 
+    POOL_BACKOFF = "poolBackoff"
+    """The pool is in backoff mode."""
+
 
 class ConnectionCheckOutFailedReason:
     """An enum that defines values for `reason` on a