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
Yes! Higher order functions *all* of the things! Maybe consider something like "server_pred", though:

1. "filter" reminds me of the HOF, not the predicate you pass to the HOF
2. single letters aren't the best, but I guess here' it's okay

(non-blocking)