Erlang's VM comes to mind. It has great APIs for inspecting a running system and quite a few options for consuming that data: build-in observer [0], CLI version [1], web live dashboard [3] and more.

[0] https://erlang.org/doc/apps/observer/observer_ug.html

[1] https://github.com/zhongwencool/observer_cli

[2] https://github.com/phoenixframework/phoenix_live_dashboard