To my surprise, Power BI only lets you put multiple values on columns in a matrix. You can’t stack metrics vertically. Note: this is true as of 8 Jan 2016 but may change in the future. If you agree that this should be a feature in Power BI, please make your voice heard and vote for this idea on the Power BI forum and encourage others to vote for it as well.
The answer is a little complex. Considering how frequently Power BI gets updated, hopefully they’ll make this a bit easier in the near future.