lds: Add listener_filter_chain.

api/lds.proto

@@ -103,6 +103,13 @@ message Listener {
 
   // The type of draining to perform at a listener-wide level.
   DrainType drain_type = 8;
+
+  // Listener filters have the opportunity to manipulate and augment the connection metadata that
+  // is used in connection filter chain matching, for example. These filters are run before any in
+  // :ref:`filter_chains <envoy_api_field_Listener.filter_chains>`. Order matters as the filters
+  // are processed sequentially right after a socket has been accepted by the listener, and before
+  // a connection is created.
+  repeated Filter listener_filters = 9;
 }
 
 message Filter {