track number of entries in index. fix #318
track number of entries in index. fix #318

raintank/metrictank,raintank/raintank-metric,raintank/raintank-metric,replay/metrictank,raintank/metrictank,raintank/metrictank,replay/metrictank,replay/metrictank,raintank/raintank-metric,raintank/metrictank,replay/metrictank