Implement a Broker metrics endpoint

[TKussel]

The Broker could serve a (HTTP Basic Auth protected) metrics endpoint, providing information such as:

• Memory Usage
• Number of tasks in queue
• Tasks/Results stratified by ProxyId and/or Metadata
• Mean Time between Task and Result creation (filtered/stratified by Metadata)

The Prometheus crate might be a good candidate for implementing the reports.

[lablans]

This is being worked on in #104

[TKussel]

I think this issue is different from the information in the health endpoint and corresponds more closely to the work in the feature/monitor branch.