@Test void retain10Percent() {
    float sampleRate = 0.1f;
    CollectorSampler sampler = CollectorSampler.create(sampleRate);

    assertThat(lotsOfSpans().filter(s -> sampler.isSampled(s.traceId(), false)).count())
        .isCloseTo((long) (LOTS_OF_SPANS.length * sampleRate), withPercentage(3));
  }
Math.abs("8000000000000000") returns a negative, we coerse to "7fffffffffffffff" to avoid
always dropping when trace_id == "8000000000000000"
zipkin-collector/core/src/test/java/zipkin2/collector/CollectorSamplerTest.java