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Kafka exposes many pluggable API for users to bring their custom plugins. For complex and critical plugins it's important to have metrics to monitor their behavior. Plugins wanting to emit metrics can use the Metrics class from the Kafka API but when creating a new Metrics instance it does not inherits the tags from the component it depends on (for example from a producer for a custom partitioner), or the registered metrics reporters. As most plugins are configurable, a workaround is to reimplement the metric reporters logic and in some case for tags too but that is cumbersome.

This issue also applies to connectors and tasks in Kafka Connect. For example MirrorMaker2 creates its own Metrics object and has logic to add the metric reporters from the configuration.

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For client and server side plugins, I propose introducing a new interface: Monitorable. If a plugin implements this interface, the withMetrics() method will be called when the plugin is instantiated (after configure() if the plugin also implements Configurable). This will allow the plugin to adds its own metrics to the existing Metrics instance from the component (producer, consumer, etc) that initiated instantiated it.

package org.apache.kafka.common.metrics;

public interface Monitorable {

    /**
     * Get the Metrics instance from the client that instantiates the plugin.
     */
    void withMetrics(Metrics Metrics);

}

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When instantiating a class via the Utils.newInstance() helper methods, if it implements Monitorable and a Metrics object is available, withMetrics() will be called with the current Metrics instance. It will be always called after configure().  Metrics registered by plugins will inherit the prefix/namespace from the current Metrics instance, these are: kafka.producer, kafka.consumer, kafka.connect, kafka.streams and kafka.server. Metrics reporters will be able to should not implement the Monitorable interface as they are created before the Metrics instance.

MirrorSourceConnector and MirrorCheckpointConnector currently register metrics using the kafka.connect.mirror prefix/namespace. If we update them with this proposal, their metrics will be renamed, for example from kafka.connect.mirror:type=MirrorSourceConnector to kafka.connect:type=MirrorSourceConnector. This KIP will not update themthese 2 connectors.

Compatibility, Deprecation, and Migration Plan

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Rejected Alternatives

• Create a dedicated a Metrics instance for plugins: A dedicated instance could have its own prefix/namespace (for example kafka.consumer.plugins). This would allow grouping metrics from all plugins but it requires instantiating another Metrics instance and new metrics reporters.
  
• Let plugins create their own Metrics instance: Instead of passing the Metrics instance to plugins we could pass all the values necessary (metrics reporters, configs, etc ...) to create and configure a Metrics instance. This is impractical as it requires passing a lot of values around and plugins still have to have logic to use them.
• Create a simpler PluginMetrics API: Instead of passing Metrics, a simpler API could be easier for plugins. It would also could prevent plugins calling close() or removeMetrics() but since plugins execute code, administrators should only use plugins they trust.