Add percent-change histogram to metrics diff report

[arnaud-lacurie]

Appends a text histogram of the % change distribution to each field's stats block in the metrics diff report.

• MetricsStatistics: collect percent diffs per field in Builder, expose as sorted FieldStatistics.sortedPercentDiffs
• MetricsDiffAnalyzer: render a 2%-wide bin bar chart after each field summary; guard against lo == hi edge case (all values identical on a bin boundary)

[github-actions]

📊 Metrics Diff Analysis Report

Summary

• New queries: 0
• Dropped queries: 0
• Plan changed + metrics changed: 0
• Plan unchanged + metrics changed: 16

ℹ️ About this analysis

This automated analysis compares query planner metrics between the base branch and this PR. It categorizes changes into:

• New queries: Queries added in this PR
• Dropped queries: Queries removed in this PR. These should be reviewed to ensure we are not losing coverage.
• Plan changed + metrics changed: The query plan has changed along with planner metrics.
• Metrics only changed: Same plan but different metrics

The last category in particular may indicate planner regressions that should be investigated.

Only Metrics Changed

These queries experienced only metrics changes without any plan changes. If these metrics have substantially changed,
then a planner change has been made which affects planner performance but does not correlate with any new outcomes,
which could indicate a regression.

Total: 16 queries

Statistical Summary (Only Metrics Changed)

task_count:

• Average change: +500.0
• Average regression: +500.0
• Median change: +500
• Median regression: +500
• Standard deviation: 0.0
• Standard deviation of regressions: 0.0
• Range: +500 to +500
• Range of regressions: +500 to +500
• Queries changed: 16
• Queries regressed: 16

There were no queries with significant regressions detected.

Minor Changes (Only Metrics Changed)

In addition, there were 16 queries with minor changes.