[Bug] Fix PubsubUnboundedSink not following 1000 messages per batch limit

[userzhy]

Fix PubsubUnboundedSink's WriterFn not enforcing message count limit per publish batch

Description

This PR fixes a bug where PubsubUnboundedSink's WriterFn was not properly enforcing the message count limit per publish batch. The publishBatchSize parameter was being passed to the constructor but was never stored or used in the processElement method.

According to Google Cloud Pub/Sub resource limits, a single publish request can contain at most 1000 messages. Without this fix, batches could exceed this limit, causing Pub/Sub to reject the publish request.

Changes

• Add publishBatchSize field to WriterFn class
• Store publishBatchSize in both WriterFn constructors
• Add message count check in processElement alongside existing byte size check
• Update both PubsubSinkDynamicDestinations and PubsubSink to pass publishBatchSize when creating WriterFn instances

Testing

• Existing PubsubUnboundedSinkTest tests pass
• The test sendMoreThanOneBatchByNumMessages specifically validates batch splitting by message count

Fixes #36885

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[gemini-code-assist]

Summary of Changes

Hello @userzhy, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical bug in the PubsubUnboundedSink component where it failed to respect Google Cloud Pub/Sub's message count limit per publish batch. By integrating the publishBatchSize parameter into the WriterFn's batching logic, the system will now proactively split message batches based on both message count and byte size, ensuring compliance with Pub/Sub's resource limits and preventing potential publish failures. Additionally, a minor enhancement was made to Python's logging mechanism to include logger names in log entries for better debugging.

Highlights

• Pub/Sub Batch Limit Enforcement: Fixed a bug in PubsubUnboundedSink where the WriterFn was not properly enforcing the 1000 messages per batch limit for Google Cloud Pub/Sub, which could lead to rejected publish requests.
• WriterFn Parameter Usage: The publishBatchSize parameter is now correctly stored and utilized within the WriterFn's processElement method to split batches by message count, complementing the existing byte size check.
• Python Logging Enhancement: The FnApiLogRecordHandler in Python now includes the logger name in its custom_data field, improving log filtering and debugging capabilities.

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