Add a warning log and more comments

example-config.yaml

@@ -284,8 +284,10 @@ bridge:
     # Null means there's no enforced timeout.
     message_handling_timeout:
         # Send an error message after this timeout, but keep waiting for the response until the deadline.
+        # This is counted from the origin_server_ts, so the warning time is consistent regardless of the source of delay.
         error_after: 10s
         # Drop messages after this timeout. They may still go through if the message got sent to the servers.
+        # This is counted from the time the bridge starts handling the message.
         deadline: 60s
 
     # The prefix for commands. Only required in non-management rooms.

portal.go

@@ -3156,6 +3156,8 @@ func (portal *Portal) HandleMatrixMessage(sender *User, evt *event.Event) {
 		if remainingTime < 0 {
 			go ms.sendMessageMetrics(evt, fmt.Errorf("%w (message is %s old)", errTimeoutBeforeHandling, messageAge), "Timeout handling", true)
 			return
+		} else if remainingTime < 1*time.Second {
+			portal.log.Warnfln("Message %s was delayed before reaching the bridge, only have %s (of %s timeout) until delay warning", evt.ID, remainingTime, portal.bridge.Config.Bridge.MessageHandlingTimeout.ErrorAfter)
 		}
 		go func() {
 			time.Sleep(remainingTime)