IGNITE-14774 Implement metrics for in-memory index pages

[sashapolo]

Desciption

https://issues.apache.org/jira/browse/IGNITE-14774

1. PageMetrics - an aggregation of all page-related metrics. At the moment of writing, contains the total number of pages and the number of index pages.
2. DataRegionMetricsImpl now contains page metrics related to every created cache group in a form of an immutable copy-on-write IntMap. It also contains an aggregation over these metrics that can be used as metrics for the whole data region.
3. Index pages allocation is tracked through the PageIO#initNewPage method, which can later be used to track other kinds of pages.
4. Index pages de-allocation is tracked in various places like replacing a page or adding it to a free list as I couldn’t find a single suitable place for this logic.

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