[Feature Request] Filter retained messages

[richcrabtree]

I would like to be able to filter out retained messages. As it is now I must publish to a separate non-retained topic for topics subscribed by mqttclpro. If I do subscribe to a topic that has a retained message, the result is the rule firing each time mqttclpro reconnects to the broker.

I believe the broker informs the client if the message on a particular topic is the retained value or is a new value as evidenced by the msg.retain var in node.red, which is set to true when the initial retained message is delivered and is undefined otherwise. This would be better addressed in mqttclpro by offering an additional option in the place where you set up the vars and filters to block retained messages. Could be a simple checkbox. Alternately the retained status as well as other metadata could be passed in an additional var.