metadata missing in action_session_start

[rgstephens]

Rasa version: 1.9.4

Rasa SDK version (if used & relevant): 1.9.0

Rasa X version (if used & relevant): not used

Python version: 3.7

Operating system (windows, osx, ...): Debian & OSX

Issue:
When metadata is passed in a new session, the metadata is correctly shown in the rasa core debug message but it is not passed to ActionSessionStart.

Video debug session.

ActionSessionStart code:

class ActionSessionStart(Action):
    def name(self) -> Text:
        return "action_session_start"

    @staticmethod
    def _slot_set_events_from_tracker(
        tracker: "DialogueStateTracker",
    ) -> List["SlotSet"]:
        """Fetch SlotSet events from tracker and carry over key, value and metadata."""

        from rasa.core.events import SlotSet

        return [
            SlotSet(key=event.key, value=event.value, metadata=event.metadata)
            for event in tracker.applied_events()
            if isinstance(event, SlotSet)
        ]

    async def run(
        self,
        dispatcher: CollectingDispatcher,
        tracker: Tracker,
        domain: Dict[Text, Any],
    ) -> List[EventType]:

        logger.info(f"run action_session_start")
        # the session should begin with a `session_started` event
        events = [SessionStarted()]

        # any slots that should be carried over should come after the
        # `session_started` event`
        events.extend(self._slot_set_events_from_tracker(tracker))

        # Grab slots from metadata
        message_metadata = []
        for e in tracker.events[::-1]:
          # Does this tracker event have metadata?
            if "metadata" in e and e["metadata"] != None:
                message_metadata = e["metadata"]
                # Does this metadata have slots?
                if message_metadata and "slots" in message_metadata:
                    for key, value in message_metadata["slots"].items():
                        logger.info(f"{key} | {value}")
                        if value is not None:
                            events.append(SlotSet(key=key, value=value))
                    break
        if len(message_metadata) == 0:
            logger.warn(f"session_start but no metadata, tracker.events: {tracker.events}")

        # an `action_listen` should be added at the end as a user message follows
        events.append(ActionExecuted("action_listen"))

        return events