Build tracer with customized sampler object not just by sampling rate

[buxingzhe]

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Currently, trace_opts.go provides a function to set sampler with signature func WithSampler(samplingRate float64) TracerOption . I hope it can provide another function like func WithSampler(sampler Sampler) TracerOption, then I can set my customized sampler.
At the same time, the RandomSampler has a little bug:

func (s *RandomSampler) IsSampled(operation string) bool {
	return s.threshold >= s.rand.Intn(100)
}

It should be s.threshold > s.rand.Intn(100).

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