code: public Kafka property(String key, String value) {
  		Preconditions.checkNotNull(key);
  		Preconditions.checkNotNull(value);
  		if (this.kafkaProperties == null) {
  			this.kafkaProperties = new HashMap<>();
  		}
  		kafkaProperties.put(key, value);
  		return this;
  	}
docs: Adds a configuration properties for the Kafka consumer.
  
  @param key property key for the Kafka consumer
  @param value property value for the Kafka consumer