feat: add parts_searched metrics for FTS

[BubbleCal]

No description provided.

[github-actions]

PR Review: feat: add parts_searched metrics for FTS

Overall: Clean, well-implemented feature that adds partitions_searched metrics for Full-Text Search, consistent with existing KNN metrics patterns.

Issues

P1: Missing metrics recording in BooleanQueryExec

The BooleanQueryExec struct at line 906+ also uses FtsIndexMetrics but doesn't call record_parts_searched(). This creates inconsistency where some FTS query types report partition metrics and others don't. Consider adding the metric recording to BooleanQueryExec::execute() as well, or document why it's intentionally omitted.

Minor Observations

• The test is comprehensive and validates both the metric value and its presence in the plan analysis output.
• The implementation correctly reuses the existing PARTITIONS_SEARCHED_METRIC constant from lance_datafusion::utils, maintaining consistency with KNN indexing.

No other significant issues found.

[codecov]

Codecov Report

❌ Patch coverage is 99.35065% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/io/exec/fts.rs	99.33%	1 Missing ⚠️

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