Add node_pool to blockedEval metric

[allisonlarson]

Description

Adds the node_pool to the blockedEval metrics that get emitted for
resource/cpu, along with the dc and node class.

Testing & Reproduction steps

The node_pool has been added to the automated tests, and seen when using manual tests and observing metrics emitted.

Links

Fixes #25933

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[tgross]

This is a probably minor thing, but all the FailedTGAllocs for a given evaluation will belong to the same node pool, because an evaluation is for a specific job and a job can only exist in a single node pool.

Rather than repeating the node pool name on the AllocMetrics structs, maybe we should just stick the node pool field on the Evaluation itself? If we wrote the field whenever we created the eval, we could potentially use that in the future for some of the zany ideas we've knocked around like per-pool scheduler workers.

[allisonlarson]

Neat, yeah that sounds good to me! I wasn't sure that Evaluation would be the right place to add a field that was only going to be used in a metric label, but if there are possible future applications I'd be happy put it there.

[aimeeu]

@allisonlarson The website/content/docs/operations/metrics-reference.mdx is now website/content/docs/reference/metrics.mdx
Please let me know if you want me to resolve the conflict and push a commit.