Metrics collection

This script will collect a DC's total desired and actual when run. 

**TODO**:
- [ ] Collect pending also
- [ ] Tests
- [ ] Dump the data to Cloud metrics
I think this function should always filter out non-group servers since its *purpose* is to get servers belonging to scaling groups. An extra callable arg that does the filtering based on status (or any other field) should solve any purpose. Although I haven't thought of `changes-since` part yet.