Metrics collection

This script will collect a DC's total desired and actual when run. 

**TODO**:
- [x] Collect pending also
- [x] Tests
- [ ] Dump the data to Cloud metrics
You get away with not checking if metadata exists because you're relying on has_group_id to have filtered them out by then? In that case; yay, although I guess the order in which the composed thing applies ops may be a little confusing for people unfamiliar with it... That said, code you don't understand is confusing, we can't fix that one :) +1