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
I guess it would be nifty if these were transducers instead of curried eager filter, because then you wouldn't even pay for the edge case. However, making sfilter just `lambda s: s` by default also covers it, of course :)