public Builder groupId(String groupId) {
      if (groupId == null) throw new NullPointerException("groupId == null");
      properties.put(GROUP_ID_CONFIG, groupId);
      return this;
    }
The consumer group this process is consuming on behalf of. Defaults to "zipkin"
zipkin-collector/kafka/src/main/java/zipkin2/collector/kafka/KafkaCollector.java