feature request: Retries middleware to set max_retries as an option

[giuppep]

As discussed in #425 it is sometimes useful to do some special handling using the on_failure callback after the last retry. However, at the moment, the on_failure callback does not have any way to dynamically determine if the current retry is the last.

In other words, I think it could be useful to either pass the max_retries information in the message options or add another callback on_last_failure.

Unless I'm missing something, I think the first option should be fairly straightforward to implement by adding a before_enqueue method to the Retries middleware

class Retries(Middleware):
    ...
    def before_enqueue(self, broker, message, delay):
        actor = broker.get_actor(message.actor_name)
        max_retries = message.options.get("max_retries") or actor.options.get(
            "max_retries", self.max_retries
        )
        message.options["max_retries"] = self.max_retries

If you think this could be a useful feature and that the proposed solution makes sense, I'd be happy to open a PR with the change.

---

As a workaround I'm currently retrieving the actor in the on_failure handler

@dramatiq.actor
def my_callback(message_data, exception_data):
    actor = broker.get_actor(message_data["actor_name"])
    max_retries = actor.options.get("max_retries", 0)

    retries = message_data["options"]["retries"]
    if retries > max_retries:
        print("handling the error")