Good work Mete!
There is still something I believe the patch did not address yet: RepositoryStatisticsImpl implements Iterable.
This contract implies that if I call #iterator() I'll get back all the existing entries.
I would expect that if I define a custom entry, it should still be returned in the #iterator() call, otherwise some consumers of the api will not be able to see all the existing TimeSeries.
Another minor issue is a method in the RepositoryStatisticsImpl: private Type getType(String type)
I think this would be better located in the Type enum, as RepositoryStatisticsImpl doesn't really need to deal with identifying a Type by its name.
I find it interesting that there are no unit tests in the patch. One test case could very well be the #iterator() problem described earlier.