The approach for metrics in agile is often different because of the intent you are using to measure things. Agile metrics need to be collected from the system without being used as a tool to drive behavior. The reason for this is simple. Most people doing agile or scrum development are using it for highly creative acts. The common phrase you get what you measure shows up big time here. People will optimize their behavior to support the metric but, when this happens the creativity plummets.
Read Perfect Planning for a deeper discussion of this topic.
Creative acts are exactly that, new creative and cannot be measured for ahead of time because we don’t know what they are. Use an agile scorecard to understand velocity and commitment based planning.