add node_status and node_scheduling_eligibility labels to client_alloc_allocated metrics

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### Nomad version
Output from `nomad version`
Nomad v0.10.4+ent

### Operating system and Environment details
Debian GNU/Linux 9.9 (stretch)

### Issue
The addition of node_status and node_scheduling_eligibility (#6130) is a great step, but in order for us to properly monitor cluster usage and scheduling availability, it would be incredibly useful to add these labels to the metrics for client allocated resources. For example, we have a grafana charting the % of available cpu resources in our cluster - however, we currently have no way to account for what hosts are ineligible/down in that chart. If we could get those labels added to the following metrics, it would be a huge help:

- nomad_client_allocated_cpu
- nomad_client_unallocated_cpu
- nomad_client_allocated_memory
- nomad_client_unallocated_memory
- nomad_client_allocated_disk
- nomad_client_unallocated_disk
- nomad_client_allocated_network
- nomad_client_unallocated_network

Thank you!

Start by tracing the eight nomad_client_allocated_* and nomad_client_unallocated_* metrics in the Nomad codebase, and review the earlier node_status and node_scheduling_eligibility change in #6130. Done means all listed CPU, memory, disk, and network metrics expose both requested labels and the resulting values support the described monitoring use case.

Written by the indexing model from the issue text.