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
Yeah. Sorry the immutable args thing is same as in `StubTreq`. It uses `freeze` before storing the requests. The main change is not providing response objects when instantiating since the tests always expect a particular response code and data for a particular request. Also, it allows multiple response (code,data) for same request. See https://github.com/rackerlabs/otter/blob/metrics/otter/test/test_convergence.py#L61