fix(*) don't crash on throughput when duration is null

[vbarbaresi]

I experienced this issue while running clearly tests

    def test_server_filter_workers_empty(mocked_server):
        mocked_server.listener.memory.workers.values.return_value = ()

        gen = mocked_server.filter_workers(clearly_pb2.FilterWorkersRequest(), N
one)
        with pytest.raises(StopIteration):
>           next(gen)

test_server.py:129:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
..\..\venv\lib\site-packages\clearly\server.py:153: in filter_workers
    for worker in about_time(callback, found_workers):
..\..\venv\lib\site-packages\about_time\about_time.py:91: in counter
    fn(HandleStats(timings, i + 1))
..\..\venv\lib\site-packages\clearly\server.py:151: in callback
    t.count, t.duration_human, t.throughput_human)
..\..\venv\lib\site-packages\about_time\about_time.py:151: in throughput_human
    value = self.throughput
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <about_time.about_time.HandleStats object at 0x03FAD530>

    @property
    def throughput(self):
>       return self.count / self.duration
E       ZeroDivisionError: float division by zero

..\..\venv\lib\site-packages\about_time\about_time.py:147: ZeroDivisionError
===================== 2 failed, 15 passed in 0.25 seconds =====================

With the mocks it was so fast that duration was actually zero. And probably because I'm currently on a Python2.7 + Windows system where the time() precision is crappy.

So I propose returning NaN in this case

[rsalmei]

Hello @vbarbaresi!
I'm sorry for the long delay to reply to you.

Thank you for catching that, and already proposing a PR!
I did a small research and you are right, the timer resolution in py2.7 windows is more than crappy, is a whopping 💩16ms! 😱

Well, I did fix it in this scenario, but I've made a more involved solution, could you please pip install -U about-time and see if it is ok for you? The changes are at #2.

Also, could you see if you can now run the tests for Clearly?

[vbarbaresi]

Thanks! Yes, your solution works for me 👌