We track Haskell threads. edit: Found the code. :) We rolled our own implementation. Our WAI application action increments a counter and decrements it again

We track Haskell threads. edit: Found the code. :) We rolled our own implementation. Our WAI application action increments a counter and decrements it again whenever an HTTP request is received and completed. It doesn't track threads created as a part of HTTP request handling, but we don't allow those actions to forkIO anyway. There hasn't been any demand for it.