def saveRegistry(self, registry=REGISTRY):
return copy.deepcopy(list(registry.collect()))<EOL>
Freezes a registry. This lets a user test changes to a metric
        instead of testing the absolute value. A typical use case looks like:

          r = self.saveRegistry()
          doStuff()
          self.assertMetricDiff(r, 1, 'stuff_done_total')