Better thread metrics

[pglombardo]

We use threading.enumerate to get a list of threads and currently report a count of dead threads but enumerate only returns the list of threads that are live.

This PR updates the thread metric reporting to not attempt to calculate dead threads and to also report a count of dummy threads.

Per the Python documentation:

threading.enumerate()

    Return a list of all Thread objects currently alive. The list includes daemonic threads, dummy thread objects created by current_thread(), and the main thread. It excludes terminated threads and threads that have not yet been started.