public Builder topic(String topic) {
      if (StringUtils.isNullOrEmpty(topic)) throw new NullPointerException("topic is null or empty");
      this.topic = topic;
      return this;
    }
Queue zipkin spans will be consumed from. Defaults to "zipkin".
zipkin-collector/pulsar/src/main/java/zipkin2/collector/pulsar/PulsarCollector.java