Autoscaling containers using local request counters
The problem can easily be solved using the following components:


On the instances serving your webapp continue to monitor the number of incoming requests – and anything else you see fit.
Publish the number of incoming requests to a monitoring system. If it is not yet implemented, this step will improve your monitoring capacities, and will help you to monitor the load on each hosts as well as host balancing.
From the incoming requests, deduce an estimated required number of webaspp servers needed to serve that work load, as well as the difference between the actual number and the estimated needed number. In the case you describe, it seems that the estimated number is just a scalar function of the total number of incoming requests on a recent period of time. On other systems or after some times, more subtle strategies can be implemented. Monitoring these quantities and the difference could ease the traceability of the auto-scaling strategy, and will monitor its responsivity.
Last implement the auto-scaling itself, at this point, this is really just reading some number from your monitoring system and writing it to your scaling system.