Add metrics counters and gauges for the follow set of metrics, and ensure they are exposed on the shipper's HTTP monitoring interface under /debug/vars:

• Report the following set of queue metrics:
  • The current configured maximum size of the queue:
    • Report the maximum queue size in events for the memory queue:

      elastic-agent-shipper/monitoring/queuemon.go

      Line 131 in e231e51

      MaxLevel: opt.UintWith(limit),

    • Report the maximum queue size in bytes for the disk queue.
  • The current utilization of the queue in events:
    • Report the current utilization of the queue in events for the memory queue:

      elastic-agent-shipper/monitoring/queuemon.go

      Line 130 in e231e51

      CurrentLevel: opt.UintWith(count),

    • Report the current utilization of the queue in bytes for the disk queue.
  • Implement queue metrics for the disk queue #145

The eventual goal is to expose these metrics on the agent details page (example https://$kbana/app/fleet/agents/$agentID) in fleet via the agent monitoring metrics.