8368501: Shenandoah: GC progress evaluation does not use generation

[earthling-amzn]

Remove unused generation parameter from stop-the-world GC progress evaluation. Also, have shMetrics use the freeset for usage details. This is a follow up to a comment from an unrelated PR: #27456 (comment).

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• JDK-8368501: Shenandoah: GC progress evaluation does not use generation (Enhancement - P4)

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