Add a queue utilization metric to report how full the queue is as a percentage

[cmacknz]

Add a new queue utilization metric to report how full the queue is as a percentage. The queue utilization should be calculated as (CurrentLevel / MaximumLevel) * 100 where the level variables are defined by:

elastic-agent-shipper/monitoring/reporter/reporter.go

Lines 19 to 22 in fff35d3

	// CurrentLevel reports the current fill state of the queue, in the native user-configured limits of the queue
	CurrentLevel opt.Uint `struct:"current_level,omitempty" json:"current_level"`
	//MaxLevel reports the user-configured max level of the queue, in the native user-configured limiits
	MaxLevel opt.Uint `struct:"max_level,omitempty" json:"max_level"`

Acceptance Criteria:

• A queue utilization metric is available on the shipper's metrics HTTP interface under the /debug/vars path.
• A test exists proving that the queue utilization changes as expected as the queue fills up. An empty queue reports 0, a half-full queue reports 50, and a full queue reports 100.