I like me the most popular answer which is: Have a little fun. I had the same idea (ie PID loop to control system behaviour) some time ago to control traffic

I like me the most popular answer which is: Have a little fun. I had the same idea (ie PID loop to control system behaviour) some time ago to control traffic to external system so that it is just at the limit of throughput of that external system. The external system is really badly implemented and we had to put much effort into maximizing throughput. In the end we are replacing HTTP requests with Kafka topics which let's the other system process as fast as possible without destabilizing. On our end we continuously monitor latency and have flow control based on consumer behaviour.