public Builder bootstrapServers(String bootstrapServers) {
      if (bootstrapServers == null) throw new NullPointerException("bootstrapServers == null");
      properties.put(BOOTSTRAP_SERVERS_CONFIG, bootstrapServers);
      return this;
    }
The bootstrapServers connect string, ex. 127.0.0.1:9092. No default.
zipkin-collector/kafka/src/main/java/zipkin2/collector/kafka/KafkaCollector.java