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
> Is it possible to unify the two classes though, rather than have a second one?

Yes. If everyone thinks this is good, it should be one class. I wanted to try it out before making changes in other tests. Do you think another PR to merge the two classes would help?