I want to configure istio in such a way that it does not uses Prometheus or Grafana which come by default with it. I want to use my existing Prometheus and Grafana which is already deplyoed in cluster. Any help will be appreciated.
You need to configure your existing prometheus with a scrape configuration.For prometheus config you can use this ConfigMap. For grafana you need to configure your prometheus as datasource and you can use this configMap for that. You can generate the ConfigMaps using helm template and use.
Check this guide