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Motivation

With the delivery of KIP-714, cluster operators can now pull metrics from a broker (sent by clients) via a plugin. KIP KIP-714 dramatically simplifies observing client behavior via metrics.  Still, it only accounts for metrics from clients (admin, consumer, and producer), leaving out applications that embed client instances, such as Kafka Streams. While the importance of capturing client metrics is evident, there should be a mechanism for capturing the enclosing application metrics, as reviewing both sets will be required to get a complete performance picture. This proposal aims to provide a mechanism for applications containing Kafka clients to supplement the client metrics with some application-specific metrics.

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The methods registerMetricForSubscription and ungregisterMetricFromSubscription  will be added to the Admin, Consumer,  and Producer  interfaces. Because of the ad hoc connections of the AdminClient this KIP will also set the default value of the AdminClient.ENABLE_METRICS_PUSH_CONFIG to false.

package org.apache.kafka.clients.admin;

public interface Admin extends AutoCloseable {  

/**
 *  An application metric provided for subscription.
 *  This metric will be added to this client's metrics
 *  that are available for subscription and sent as 
 *  telemetry data to the broker.
 *
 * @param metric, the application metric to register
 */ 
void registerMetricForSubscription(KafkaMetric metric)

/**
 *  An application to be removed from subscription.
 *  This metric is removed from this client's metrics
 *  and will not be available for subscription.
 *
 * @param metric, the application metric to remove
 */ 
void unregisterMetricFromSubscription(KafkaMetric metric)

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