Stats callback in promisified api

[emasab]

Statistics callback is available when using the promisified API with all client types.

Note for the reviewers: SetPollInBackground is moved to the Connection base class, false by default and it's public only on the Producer where it's possible to chose if polling externally. For the AdminClient it's enabled only if the statistics.interval.ms is set as the main reply queue wasn't polled.

Checklist

• Contains customer facing changes? It's possible to set the statistics.interval.ms together with the stats_cb and receive the statistics
• Did you add sufficient unit test and/or integration test coverage for this PR? Unit tests are present for the three client types

[Copilot]

Pull Request Overview

This PR enables statistics callback support in the promisified API for all client types (Producer, Consumer, and Admin). Previously, stats callbacks were only available in the non-promisified API. The implementation moves background polling setup from Producer to the base Connection class, allowing all client types to receive statistics events.

Key Changes:

• Refactored SetPollInBackground from Producer-specific to Connection base class
• Added stats callback configuration and event handling for Producer, Consumer, and Admin clients
• Added comprehensive test coverage for stats callbacks across all three client types

Reviewed Changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/connection.h	Added SetPollInBackground method declaration and m_is_background_polling field to base Connection class
src/connection.cc	Implemented SetPollInBackground method in Connection class with background polling queue management
src/producer.h	Changed to use inherited SetPollInBackground from Connection class
src/producer.cc	Removed Producer-specific SetPollInBackground implementation and m_is_background_polling field
src/admin.cc	Added automatic background polling activation when statistics interval is configured
lib/kafkajs/_producer.js	Added stats callback extraction, storage, and event listener registration
lib/kafkajs/_consumer.js	Added stats callback extraction, storage, and event listener registration
lib/kafkajs/_admin.js	Added stats callback extraction, storage, and event listener registration
test/promisified/producer/statsCallback.spec.js	Added test verifying Producer receives stats callbacks
test/promisified/consumer/statsCallback.spec.js	Added test verifying Consumer receives stats callbacks
test/promisified/admin/statsCallback.spec.js	Added test verifying Admin receives stats callbacks
CHANGELOG.md	Documented new statistics callback feature for promisified API

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