public Builder sampler(CollectorSampler sampler) {
      if (sampler == null) throw new NullPointerException("sampler == null");
      this.sampler = sampler;
      return this;
    }
Sets {@link {@link CollectorComponent.Builder#sampler(CollectorSampler)}}
zipkin-collector/core/src/main/java/zipkin2/collector/Collector.java