This class would populate some statistics from a reader. We want to be careful here such that
we only collect statistics which don't incur reads from the index i.e. we would only collect
stats that lucene would already have read into memory when the reader was opened.
field - Index field for which number of indexed documents are to be return
number of indexed documents (without subtracting potentially deleted ones)
for the given field.
-1: if index codec doesn't store doc-count-for-field statistics, OR
reader threw an exception while reading fields, OR
exception thrown while reading count for the field, OR
doc-count is asked for a non-property field.