Is there any documentation and information about what a derived metric is and what it can do?
From the Foglight Administration and Configuration Guide:
"A metric is a specific value that is measured over time. There are two types of metrics in Foglight: raw metrics and derived metrics. Raw metrics are collected directly from your monitored environment and sent to the Foglight Management Server. Derived metrics are calculated from one or more raw or derived metrics. They are scoped to a topology type and can optionally be scoped to specific objects of that type. Many derived metrics come included with Foglight and installed cartridges. If none of the existing derived metrics meet your needs, you can create a new one and add it to the derived metric collection."
Please find more information in the Guide and the online help in the Foglight web console.