LUCENE-3555 added explicit stats methods to indexsearcher, but you must
subclass to override this (e.g. populate with distributed stats).
Its also impossible to then do this with SearcherManager.
One idea is make this a factory method (or similar) on IndexSearcher instead,
so you don't need to subclass it to override.
Then you can initialize this in a SearcherWarmer, except there is currently
a lot of hair in what this warming should be. This is a prime example where
Searcher has different meaning from Reader, we should clean this up.
Otherwise, lets make NRT/SearcherManager subclassable in such a way that
you can return a custom indexsearcher.