Feature Request: Add specification-based metrics

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If filing a bug please include the following:

### Nomad version
Nomad v0.7.1 (0b295d399d00199cfab4621566babd25987ba06e)

### Operating system and Environment details
Debian Linux 8.7

### Issue
We've been using the prometheus-exported job metrics, which have been really useful. However, it would be even more useful if we could get metrics based on the latest evaluated job specification. There may be other useful metrics, but the metrics that immediately jump to mind are:
* per-task-group group_count (to validate running allocations == intended allocations)
* per-task desired resource metrics (cpu, iops, memory, network mbits)
Memory in particular would be useful in order to determine tasks that are close to getting OOM-killed.

Start by reviewing Nomad's existing Prometheus-exported job metrics and how the latest evaluated job specification is represented. Done means exposing per-task-group group_count and per-task desired CPU, IOPS, memory, and network metrics, with any additional metrics clearly scoped.

Written by the indexing model from the issue text.