A custom statistics object. Custom statistics allow for customization
of statistics that are written at periodic intervals to the je.stats.csv
file. The field names returned from the getFieldNames() method are used as
column headers in the je.stat.csv file. The getFieldNames() method is only
called once when the environment is opened. The field values are associated
with the field names in the order of the returned array. The
getFieldValues() method is called when a row is written to the statistics
file. The semantic for the values are implementation specific. The values
may represent totals, incremental (since the last getFieldValues() call), or
stateless (computed at the time the statistic is requested).