Metrics collection

This script will collect a DC's total desired and actual when run. 

**TODO**:
- [x] Collect pending also
- [ ] Tests
- [ ] Dump the data to Cloud metrics
This is a really nice set of tests!  My only comment is a bit of a bikeshed comment, which is that the name of the tests + the docstrings aren't super clear as to what is being tested.  For instance, `no_batch` implies no results to me, or that the batch argument is not supplied.  Maybe `test_all_groups_less_than_batch`?

The filter one is great - will add comments to the next two.