RFR: 8311978: Shenandoah: Create abstraction over heap metrics for heuristics [v10]

[Paul Hohensee]

On Fri, 21 Jul 2023 17:51:57 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> This change introduces a new interface `ShenandoahHeapStats`. Presently, the interface is implemented by `ShenandoahHeap`. The generational mode for Shenandoah will provide a new implementation of this interface `ShenandoahGeneration`.
>
> William Kemper has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix names for include guards

Marked as reviewed by phh (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/14856#pullrequestreview-1541877669