public String getValue(long val) {
        if (this.valuePrefix != null) {
            return String.format("%d.%d", this.valuePrefix, val);
        }
        return String.format("%d", val);
    }
Returns a string to publish: ether 'valuePrefix'.'val' or 'val'
tools/src/main/java/org/apache/kafka/tools/VerifiableProducer.java