Observability and management for Janus job queues

[tgeoghegan]

divviup/janus#2781 discusses giving Janus interfaces for managing aggregation and collection job queues. Once these are wired up in the aggregator API, divviup-api could do stuff with them like:

• Admin consoles allowing DU ops to manage queues
• Expose job backlog metrics to subscribers

While operators can technically do this by directly tickling the Janus aggregator API, it's much nicer to have this exposed in divviup-api, where we can gate access on Auth0 users.