Why not use the Auto-Scaling Groups implementations that are already there?  First, auto-scaling group implementations vary from one cloud provider to the next, if even available.  Second, most auto-scalers are designed to manage cattle, where individual instances in a group are identical to one another.  This is clearly not the case for the Swarm managers:

via InfraKit and Docker Swarm Mode: A Fault-Tolerant and Self-Healing Cluster – Docker Blog