currently we can set time period in days for which to keep item values - anything older gets nuked by the housekeeper.

there could be items that get values infrequently and at nearly unpredictably times - maybe a trapper item that we are sending software version to only during the upgrade process.

for such items, we might only want to keep last n values - one, two, three etc. we would have no idea how they would end up being distributed in time - could be hours, could be years.
gathering the values all the time would be useless, increase the db size and make the data much less clear. we would have to find the value change among a huge amount of data instead of seeing a few useful values with proper timestamps.

it would be useful to set the retention for such items not to a time period, but to the number of values to be kept.