add GC stats. fix #127
add GC stats. fix #127

replay/metrictank,raintank/metrictank,OOM-Killer/metrictank,raintank/metrictank,raintank/raintank-metric,raintank/metrictank,replay/metrictank,OOM-Killer/metrictank,raintank/metrictank,OOM-Killer/metrictank,raintank/raintank-metric,raintank/raintank-metric,replay/metrictank,replay/metrictank