fix: Covered Experiences fluentd configuration

What

Fix for fluentd configuration for the config/covered_experiences.log log file. And documentation describing this configuration.

Why

This configuration failed previously, and there's no documentation yet describing what each field does and their importance.

I've create a follow up issue in the groupObservability backlog to improve this configuration in general: gitlab-com/gl-infra/observability/team#4369.