Identify possible improvements to Filebeat monitoring docs

### Description

A user reported difficulty understanding the [Filebeat monitoring documentation](https://www.elastic.co/guide/en/beats/filebeat/current/monitoring.html). The current documentation does not list all metrics, indicate which are counters or gauges, or provide consistent explanations. Some troubleshooting hints may also be outdated or inaccurate.

### Resources

Here is the post: https://discuss.elastic.co/t/missing-metrics-documentation/383047

### Which documentation set does this change impact?

Elastic On-Prem only

### Collaboration model

The documentation team