There are  multiple timestamps if more than one metric was received within the time period. However, that does leave the part that if metrics for a single key show up one per interval, then the slope will always be 0. I think this is kind of a killer for this. We have to do the trick with the calculated metric where we look at the previous interval for the slope if there is only one datapoint.  
I'll close this and we'll go with the calculated metric.  Unless we switch to graphite which does this for us......