How to scale the control plane of a service mesh?
It's true that when you scale your services is possible to have a bottleneck on the monitoring, but for avoiding that you have to check if your monitoring system is pull or push, in case it is push maybe is not the best option but pull is not the right answer for all situations.

One of the most used monitoring systems today is prometheus, you can se how it works here:

https://prometheus.io/docs/introduction/overview/

But is not the only one and you need to see how it should fit in your stack.

Check the possible integrations with istio

https://istio.io/docs/tasks/telemetry/metrics/querying-metrics/

In case you have already some of that probably need to think on scaling this part instead of the sidecars or istio itself.