Sample ratio not align with configured SampleRate during Stress Relief

[VinozzZ]

Description:

When Stress Relief mode is activated, a fraction of traffic is sampled through a deterministic sampler based on the hash of trace id calculated using wyhash. When observing the kept_from_stress and dropped_from_stress metrics, the ratio between the two does not always align with the SampleRate configured for stress relief

Potential Cause
Below test showed that with smaller iteration n, wyhash result sometimes can have less distribution. This can be a reason why more traces are kept than configured SampleRate

func TestWyhash(t *testing.T) {
	n := 10000
	const frac = 100
	var upperBound uint64 = math.MaxUint64 / frac
	for i := 0; i < 10; i++ {
		t.Run(fmt.Sprintf("frac=%d", frac), func(t *testing.T) {
			count := 0
			for i := 0; i < n; i++ {
				traceID := fmt.Sprintf("%016x%016x", rand.Int63(), rand.Int63())
				hash := wyhash.Hash([]byte(traceID), hashSeed)
				if hash <= upperBound {
					count++
				}
			}
			assert.InDelta(t, count, n/frac, 0.1*float64(n/frac))
		})
	}
}