logical: add initial/catchup scan metrics to offline scan

[kev-cao]

This patch teaches the LDR offline scan processor to emit checkpoint range stats and update the logical_replication.scanning_ranges and logical_replication.catchup_ranges metrics.

Informs: #152273

Release note: LDR now updates the logical_replication.scanning_ranges and logical_replication.catchup_ranges metrics during fast initial scan.

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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[cockroach-teamcity]

This change is 

[kev-cao]

Screenshots

I think some additional work could probably be done for the job messages to make it more clear that we've moved from offline scan to the steady state. Just a one-liner adding a job message when the offline processor is complete is probably sufficient.

[Copilot]

Pull Request Overview

This PR enhances the LDR offline scan processor to emit checkpoint range statistics and update the logical_replication.scanning_ranges and logical_replication.catchup_ranges metrics during fast initial scans.

Key changes:

• Added range statistics collection to the offline initial scan processor
• Refactored error handling in the Next() method for better maintainability
• Enhanced checkpoint processing to include range statistics

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
pkg/crosscluster/logical/offline_initial_scan_processor.go	Added range statistics channel, refactored error handling, and enhanced checkpoint processing to emit range stats
pkg/crosscluster/logical/logical_replication_job.go	Added tracking of write processor count for metrics

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