Document Airflow Metrics Optimal Values

**Description**

Users of Airflow appreciate the [metrics](https://airflow.apache.org/docs/apache-airflow/stable/logging-monitoring/metrics.html) that Airflow emits, however are often confused as to what values indicate a healthy system versus a system that might be exhibiting problems.

This feature request is to add value ranges to the metrics documentation as to what values are appropriate, as well as troubleshooting information as to how to address when those metrics fall outside recommended values.

**Use case / motivation**

For example, looking at `dag_processing.total_parse_time` under [Gauges](https://airflow.apache.org/docs/apache-airflow/stable/logging-monitoring/metrics.html?highlight=metrics#gauges), what are appropriate values here? As a user, how would I know that my scheduler is under-powered, or that I have too many/too complex DAGs, or that some other issue exists?

**Are you willing to submit a PR?**

Yes

**Related Issues**

N/A