Are they creating their own mesh networks for internal or user use?
Now you can simply write a script that uses the tailscale API to find all "metrics-service-*" nodes in your tailnet, and then adds their IP/DNS to your prometheus scraping list. Run it every 60 seconds. Done, now you can just deploy your app anywhere on any cloud and it will get scraped and that route will never be exposed to the outer internet.
This will basically just let you attach bespoke applications and not just "computers" to your network. I suspect I will get a lot of use from it.
Likely more transparent than explicit/implicit TLS.