private void setIfAbsent(Properties props, String key, String value) {
        if (!props.containsKey(key)) {
            props.setProperty(key, value);
        }
    }
Send a AutoBalancerMetric to the Kafka topic.

@param ccm the auto balancer metric to send.
core/src/main/java/kafka/autobalancer/metricsreporter/AutoBalancerMetricsReporter.java