metrics: classless blocked evals get metrics

[shoenig]

This PR fixes a bug where blocked evaluations with no class set would
not have metrics exported at the dc:class scope.

Fixes #13759

Also drive-by fixup naming of ByNode to ByClassInDC which is more accurate 🤦

Backport to 1.3.x, 1.2.x

[shoenig]

Spot check

➜ nomad eval status 33
ID                 = 33e76d40
Create Time        = 9s ago
Modify Time        = 9s ago
Status             = blocked
Status Description = created to place remaining allocations
Type               = service
TriggeredBy        = queued-allocs
Job ID             = example
Namespace          = default
Priority           = 50
Placement Failures = N/A - In Progress

Failed Placements
Task Group "cache" (failed to place 1 allocation):
  * Resources exhausted on 1 nodes
  * Dimension "memory" exhausted on 1 nodes

➜ napi /v1/metrics?format=prometheus | rg -v '^#' | rg blocked_eval 
nomad_nomad_blocked_evals_cpu{datacenter="dc1",host="x52",node_class=""} 0
nomad_nomad_blocked_evals_job_cpu{host="x52",job="example",namespace="default"} 0
nomad_nomad_blocked_evals_job_memory{host="x52",job="example",namespace="default"} 64000
nomad_nomad_blocked_evals_memory{datacenter="dc1",host="x52",node_class=""} 64000
nomad_nomad_blocked_evals_total_blocked{host="x52"} 1
nomad_nomad_blocked_evals_total_escaped{host="x52"} 0
nomad_nomad_blocked_evals_total_quota_limit{host="x52"} 0

[lgfa29]

Rude 😂

[schmichael]

Thanks Seth!

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