@Override
    public Builder metrics(CollectorMetrics metrics) {
      if (metrics == null) throw new NullPointerException("metrics == null");
      this.metrics = metrics.forTransport("kafka");
      delegate.metrics(this.metrics);
      return this;
    }
Configuration including defaults needed to consume spans from a Kafka topic.
zipkin-collector/kafka/src/main/java/zipkin2/collector/kafka/KafkaCollector.java