Add a metric for the queue utilization (or fill) percentage. The metric can be calculated as the current size of the queue divided by the maximum size of the queue.

This metric can then be reported to Fleet when running under the Elastic Agent and used to easily detect and flag agents that are experiencing output backpressure.

Acceptance Criteria:

• The metric exists when polling /stats and in the 30s metrics in the beat logs
• The metric is queryable from the applicable metrics-elastic_agent.* data streams in the agent package https://github.com/elastic/integrations/tree/main/packages/elastic_agent
• The metric is available in the agent metrics dashboard in Fleet. The queue size is already available, the utilization could replace the size as it is much easier to interpret.