Controller provides Agent metrics

Current Situation

• With JS-2214 each Controller instance and Agent instance offers metrics for scraping with Prometheus.
• Users have to configure each Agent individually for scraping by Prometheus.

Desired Behavior

• The active Controller provides metrics for registered Agents.
  • The same information is offered as available directly from the Agent.
  • If Agents are not coupled or are not reachable, then this is reported.
• Users can continue to directly scrape an Agent's metrics without use of the Controller.
• Grafana dashboards can be used similarly when scraping metrics per Agent and when scraping via the Controller.