Make MonitorField support M2M fields

[browser-bug]

Setting the monitor value on the MonitorField pointing to a M2M field currently throws this exception:

"<Model: Model object (None)>" needs to have a value for field "id" before this many-to-many relationship can be used.

This is due to:

    def get_monitored_value(self, instance: models.Model) -> Any:
        # Trying to access the related model instance before the instance is actually saved
        return getattr(instance, self.monitor)

Moreover there isn't a way to get a real difference between the old and new value on a M2M field.

Since the only way I though of was to leverage the m2m_changed Django signal, I extended the field this way.

class M2MMonitorField(MonitorField):
    """Extend MonitorField to also support M2M fields.

    Always set the initial value with the default.
    When the `m2m_changed` signal is triggered update the monitor field.
    """

    def contribute_to_class(
        self,
        cls: type[models.Model],
        name: str,
        *args,
        **kwargs,
    ) -> None:
        super().contribute_to_class(cls, name, *args, **kwargs)
        models.signals.post_init.connect(self._setup_m2m_signal, sender=cls)

    def get_monitored_field(self, instance: models.Model):
        return instance._meta.get_field(self.monitor)  # noqa: SLF001

    def get_monitored_value(self, instance: models.Model):
        monitored_field = self.get_monitored_field(instance)
        if monitored_field.many_to_many:
            return self.default
        return getattr(instance, self.monitor)

    def _setup_m2m_signal(
        self,
        sender: type[models.Model],
        instance: models.Model,
        **kwargs,
    ) -> None:
        monitored_field = self.get_monitored_field(instance)
        if monitored_field.many_to_many:
            sender = monitored_field.remote_field.through
            models.signals.m2m_changed.connect(self._save_m2m_field, sender=sender)

    def _save_m2m_field(
        self,
        sender: type[models.Model],
        instance: models.Model,
        **kwargs,
    ) -> None:
        value = timezone.now()
        setattr(instance, self.attname, value)
        instance.save_base(update_fields=self.attname, raw=True)

This is working as expected but I'd like to hear if there are any possible flaws or room for improvements.