Flaky test: tests/python/twitter/common/metrics/test_sampling.py:test_sampler_base

[kwlzn]

this can manifest in CI as:

                     tests/python/twitter/common/metrics/test_sampling.py F..
                     tests/python/twitter/common/metrics/test_gauges.py ......

                     ==================== FAILURES ====================
                     _______________ test_sampler_base ________________

                         def test_sampler_base():
                           class TestSampler(SamplerBase):
                             def __init__(self, period, clock):
                               self.count = 0
                               SamplerBase.__init__(self, period, clock)

                             def iterate(self):
                               self.count += 1

                           test_clock = ThreadedClock()
                           sampler = TestSampler(Amount(1, Time.SECONDS), clock=test_clock)
                           sampler.start()

                           assert test_clock.converge(threads=[sampler])
                           test_clock.assert_waiting(sampler, 1)

                           test_clock.tick(0.5)
                           assert test_clock.converge(threads=[sampler])
                           assert sampler.count == 0

                           test_clock.tick(0.5)
                           assert test_clock.converge(threads=[sampler])
                           assert sampler.count == 1

                           test_clock.tick(5)
                           assert test_clock.converge(threads=[sampler])
                     >     assert sampler.count == 6
                     E     assert 2 == 6
                     E      +  where 2 = <TestSampler(Thread-1 [TID=5876], started daemon 140429632481024)>.count

                     tests/python/twitter/common/metrics/test_sampling.py:58: AssertionError
                     -------------- Captured stderr call --------------
                     [<twitter.common.testing.clock.ThreadedClock object at 0x2dd1990>] Time now: 0.0
                     [<twitter.common.testing.clock.ThreadedClock object at 0x2dd1990>] Time now: 0.0
                     [<twitter.common.testing.clock.ThreadedClock object at 0x2dd1990>] Time now: 0.5
                     [<twitter.common.testing.clock.ThreadedClock object at 0x2dd1990>] Time now: 0.5
                     [<twitter.common.testing.clock.ThreadedClock object at 0x2dd1990>] Time now: 1.0
                     [<twitter.common.testing.clock.ThreadedClock object at 0x2dd1990>] Time now: 1.0
                     [<twitter.common.testing.clock.ThreadedClock object at 0x2dd1990>] Time now: 1.0
                     [<twitter.common.testing.clock.ThreadedClock object at 0x2dd1990>] Time now: 2.0
                     [<twitter.common.testing.clock.ThreadedClock object at 0x2dd1990>] Time now: 6.0
                      1 failed, 21 passed, 1 warnings in 0.44 seconds =

but succeeds on rebuild.