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
> Should the assumption that the list of servers has already been filtered be added to the documentation?

It currently documents arguments https://github.com/rackerlabs/otter/blob/metrics/otter/metrics.py#L139

> would it make sense to groupBy the status?

I thought so too. Then I realized the caller sends only ACTIVE or BUILD servers and filtered based on that. I also realized that this function assuming such a thing is not a good idea. Then I got lazy and left it.