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The only support is adding the IP addresses of the Grafana instance to the firewalls. You can see the IP addresses in the configuration page and click enable on that option.
After that you need to refresh the page so the portal can fetch new information from ARM in order to see the IP addresses.
To access via private endpoints, we are currently investigating but not concrete ETA yet. We are hoping to have this solved next year.
Please let me know if you have any other questions.