rabbit_feature_flags: Log when the controller is starting

[Why]
This is to be consistent with the fact that it logs something when it
exits. It also helps while debugging it.

Author: dumbbell

deps/rabbit/src/rabbit_ff_controller.erl

@@ -183,6 +183,9 @@ callback_mode() ->
     state_functions.
 
 init(_Args) ->
+    ?LOG_DEBUG(
+       "Feature flags: controller standing by",
+       #{domain => ?RMQLOG_DOMAIN_FEAT_FLAGS}),
     {ok, standing_by, none}.
 
 standing_by(