Ask HN: How do you measure a developers 'performance'?

My boss always asks for 'a number, any number' to show how the team is doing.

It makes sense since every other department has numbers to back their performance but it's tough to put one together for the engineering team.

Before you say "trusting your team is enough" ... it's really not.. My team trusts one another but its no replacement for having concrete data to:

1. Show the larger org 2. Help us improve internally 3. See the effects of our changes

Any suggestions?