[API Proposal]: PinnedBlockMemoryPool metrics

[BrennanConroy]

Background and Motivation

Add metrics to the PinnedBlockMemoryPool that's used by Kestrel, IIS, and Http.Sys.

Proposed API

System.Buffers.PinnedBlockMemoryPool

Name	Instrument Type	Unit	Description
pinnedblockmemorypool.current_memory	UpDownCounter	{bytes}	Number of bytes that are currently pooled by the pinned block memory pool.

Name	Instrument Type	Unit	Description
pinnedblockmemorypool.total_allocated	Counter	{bytes}	Total number of allocations made by the pinned block memory pool.

Name	Instrument Type	Unit	Description
pinnedblockmemorypool.evicted_memory	Counter	{bytes}	Total number of bytes that have been evicted.

Name	Instrument Type	Unit	Description
pinnedblockmemorypool.usage_rate	Counter	{bytes}	Rate of bytes rented from the pool.

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