rabbit_channel: Ignore DOWN message from monitored process if it exited normally

[Why]
It happens in CI from time to time and it was crashing the channel
process. There is always a `channel.close` method pending in the
channel mailbox.

[How]
For now, log something and ignore the DOWN message. The channel will
exit after handling the pending `channel.close` method anyway.

deps/rabbit/src/rabbit_channel.erl

@@ -36,6 +36,8 @@
 %% When a queue is declared as exclusive on a channel, the channel
 %% will notify queue collector of that queue.
 
+-include_lib("kernel/include/logger.hrl").
+
 -include_lib("rabbit_common/include/rabbit_framing.hrl").
 -include_lib("rabbit_common/include/rabbit.hrl").
 -include_lib("rabbit_common/include/rabbit_misc.hrl").
@@ -729,6 +731,10 @@ handle_info({{'DOWN', QName}, _MRef, process, QPid, Reason},
             handle_eol(QRef, State)
     end;
 
+handle_info({'DOWN', _MRef, process, Pid, normal}, State) ->
+    ?LOG_DEBUG("Process ~0p monitored by channel ~0p exited", [Pid, self()]),
+    {noreply, State};
+
 handle_info({'EXIT', _Pid, Reason}, State) ->
     {stop, Reason, State};