replace average with median for time-on-page metric

[VMois]

The average is more affected by outliners and anomalies compare to the median. Using the median will show a more accurate picture.

As of now, a simple median is enough. Collect all time-on-page numbers for a page in the array, sort them, and extract the median. In the future, in case the number of elements in the array grows too much bigger, we can use remedian algorithm to improve performance.