Added grace time setting for roomba cleanup

I had the problem, that for metrics that aren't updated every second (or more often) the roomba cleaned up entries that are older than FULL_DURATION. The result was, that without new datapoints the duration of my metric was smaller than FULL_DURATION so the INCOMPLETE exception occured. 
This change add's a setting for grace time, so that you have some datapoints that are a little bit older than FULL_DURATION.

(Although I'm not sure if I've overseen s.th. and if there's another solution to my problem.... At least it works now ^^)