Write tutorial - What does this apdex metric mean?

This is a how-to / demo style of tutorial.

Tracing a composite metric back through its recording-rule transformations, down to the original underlying raw metrics exposed by the system component being measured.

Use an apdex metric as a concrete example, since those are used in alerting rules and often leave folks wondering what the metric specifically measures (i.e. how is it calculated, what events are included/excluded).