So what could be done is to control each slot's Application Insight settings with environment variables (https://learn.microsoft.com/en-us/azure/azure-monitor/app/java-standalone-config for Java apps).

What I did is to add environment variable APPLICATIONINSIGHTS_METRIC_INTERVAL_SECONDS=<something_really_big_here> to dramatically decrease the frequency of reported metrics, hence the size of data ingested. A little bit hacky, but does the trick.