Is WiFi data usage included in the "Data Usage" screen? Android devices feature a Data usage screen, accessible from the main pull-down menu.

The data usage page lists both the total data consumed and the data consumed per application.



Is WiFi data included in this count, or only Cellular data?

P.S. It makes sense that the metered data count only includes Cellular data, but I would like to have a canonical source to settle this issue.
What you are seeing here is cellular data (mobile data) only. 

I am not sure what you would count as a "canonical answer", but the following three screen captures have been taken from a stock android ROM with Android 5.1.1 (as you can see by the presence of the Samsung bloatware):

  
Data Usage (default) / with hamburger-menu / data usage, WiFi tab (click images for larger variants)

The first screen capture is the same as yours.

The second screen capture shows the menu that pops up if you touch the kebab icon (the three vertical dots.)  As you see, the menu contains a "Show Wi-Fi usage" option.  If you enable showing Wi-Fi usage, two tabs are added to the screen, one for mobile data (probably "cellular data" in your case) and another for Wi-Fi.

The third screen capture shows how the "Wi-Fi" tab looks like on the screen.