Feature Request: Region wide resource allocation endpoint

It would be very nice to have an api endpoint that would return the amount of allocated resources across the cluster or per region. This would be useful for capacity planning and knowing when to scale. I would like to be clear, the amount of allocated resources not the amount consumed.

Yes, we could iterate across all nodes and jobs to gather and combine the information, but a single endpoint would make it significantly simpler.

Start by reviewing Nomad's existing API endpoint patterns and how allocated resources are represented across nodes, jobs, clusters, and regions. Done means a single endpoint reports allocated resources, distinct from consumed resources, for the whole cluster or an individual region.

Written by the indexing model from the issue text.