Only update the partition watermark iff the partition watermark in metadata table is smaller than the given watermark.

[jiangzzhu]

The updateWatermark is called in QueryChangeStreamAction.updateWatermarkCallback. This function update the watermark in a async way, so the update order is not guaranteed.

Updated the update watermark logic so that we only move the watermark of one partition forward.

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Summary of Changes

Hello @jiangzzhu, 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 introduces a critical refinement to the watermark management logic within the Spanner Change Streams connector for Apache Beam. By implementing a conditional update mechanism, the system now ensures that partition watermarks only advance forward, preventing potential data consistency issues that could arise from asynchronous update operations. This enhancement improves the robustness and reliability of watermark progression in stream processing.

Highlights

• Conditional Watermark Update: The updateWatermark method in PartitionMetadataDao now includes a check to ensure that a partition's watermark is only updated if the new watermark value is strictly greater than the currently stored watermark in the metadata table.
• Asynchronous Safety: This change addresses potential issues with out-of-order updates from asynchronous updateWatermark calls, guaranteeing that partition watermarks always move forward monotonically and prevent regressions.
• Improved Error Handling: Added logging to capture and report errors if the system fails to read the existing watermark from the metadata table before attempting an update.

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[tianz101]

LGTM, can you make sure Sam review and help merge? Thanks.

[scwhittle]

Fixes #36470