release-24.1: kv: add raft.commands.pending metric

[blathers-crl]

Backport 1/1 commits from #123264 on behalf of @nvanbenschoten.

/cc @cockroachdb/release

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Fixes #123259.

This commit adds a new metric called raft.commands.pending. This metric tracks the number of Raft commands that the leaseholders on a node are tracking as in-flight. These commands will be periodically reproposed until they are applied or until they fail, either unequivocally or ambiguously.

Demonstration that this is hooked up properly while running cockroach workload run kv --init --concurrency=512:

Release note: None

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Release justification: improve observability.

[blathers-crl]

Thanks for opening a backport.

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[cockroach-teamcity]

This change is