RFR: 8311978: Create abstraction over heap metrics for heuristics [v7]

[Paul Hohensee]

On Wed, 19 Jul 2023 19:24:06 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:
> 
>   Add documentation describing each heuristic

One small nit:

shenandoahHeuristics.hpp: Remove #include of shenandoahHeuristics.hpp, it's redundant.

-------------

Changes requested by phh (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/14856#pullrequestreview-1539801524