Feature Request: "debug" flag should surface Prometheus scraper errors

I assumed the agent surfaces Prometheus scraper errors in the agent logs, especifically when the agent { debug: true } is set. Turns out it doesn't. Would of been a time-saver if it did.

EDIT: There is an undocumented `DEBUG` environment variable to enable Prometheus debug logging in the CloudWatch Agent:

https://github.com/aws/amazon-cloudwatch-agent/blob/71bdd66ac78de664621fce0b3e5c78c11cd15adb/plugins/inputs/prometheus/start.go#L82-L90

[EDIT: removed confused bugreport caused by warning while expecting other errors in the logs]

Start with plugins/inputs/prometheus/start.go around lines 82-90 and trace how the DEBUG environment variable currently enables Prometheus logging. Compare that path with the agent's debug: true setting and identify the expected scraper-error output. Done means scraper errors appear in the agent logs when debug mode is enabled; verify the behavior with the relevant Prometheus input tests if available.

Written by the indexing model from the issue text.