[VSA] Add averages to VSA table to supplement medians

Problem to solve

Value Stream Analytics was originally implemented to display medians. Medians were chosen because they exclude "outliers" - extremely high or low values that skew averages. However, there are some problems with using medians:

• Unfamiliarity: Generally, people are more familiar with averages than medians, and find averages easier to understand. This is true to the extent that we have an issue about how it is unclear that metrics are medians, even though the column is clearly labeled "Median"!
• Unexpectedness: People familiar with devops analytics expect to see averages; as @hdelalic has mentioned: "cycle time, throughput, velocity, and other predictive measures are always expressed as averages" .
• Inadequacy: Although medians were chosen to exclude the influence of extreme values, we have found many cases where the majority of values are extreme. In these cases, median is worse than useless - it feels misleading. For example, there was a case where 11 of 20 items in a stage had a median of 0, so the stage median was 0. Again in the words of @hdelalic: "in sets like this one where more than a half of the values "sits" in one of the extremes (0), the median value will always be that extreme and the user will not get any valuable info out of that number".

Proposal

VSA should display an Average column to the left of the Median column.