Top 2 metrics to measure Agile/XP team health

The first is the number of defects found after development. 

An XP team should have dramatically fewer defects in its first deployment and make rapid progress from there. Some XP teams that have been on the path of improvement for several years see only a handful of defects per year. No defect is acceptable; each is an opportunity for the team to learn and improve.

The second metric is the time lag between the beginning of investment in an idea and when the idea first generates revenue. 

Even small organizations typically find they take more than a year from investment to return. Gradually reducing the time from investment to return increases the amount and timeliness of feedback available to the whole team.

Both post-development defects and investment-to-return are indicators of team effectiveness much as a speedometer is an indicator of speed. You don't make a car go faster by grabbing the speedometer needle and moving it over. If you want to go faster, you push on the gas pedal and then look at the speedometer to see if that was effective. Similarly, while you can set goals based on metrics, your team needs to address underlying problems. Trying to “game” the numbers directly will not result in improvement of anything but the numbers and defeats the value of transparency on the project.

Source - extreme-programming-explained - by Kent Beck; Cynthia Andres