fix poll period for rds metric "VolumeBytesUsed" (#1254)
fix poll period for rds metric "VolumeBytesUsed" (#1254)

It looks like rds metric "VolumeBytesUsed" is published
with a large delay up to an hour. With default period count
is 6, the look back period is 10m * 6 = 1h, plus the default
1 period offset, it seems reasonable.
copperlight/atlas,brharrington/atlas,brharrington/atlas,Netflix/atlas,Netflix/atlas,copperlight/atlas,Netflix/atlas,brharrington/atlas,copperlight/atlas