fix(measurex): use same keys of the OONI data format (#572)

This change should simplify the pipeline's job.

Reference issue: https://github.com/ooni/probe/issues/1817.

I previously dismissed this possibility, but now it seems clear it
is simpler to have a very tabular data format internally and to
convert such a format to OONI's data format when serializing.

The OONI data format is what the pipeline expects, but processing
is easier with a more linear/tabular format.

internal/tutorial/measurex/chapter09/README.md

@@ -101,6 +101,11 @@ fully measured, this method closes the returned channel.
 Like we did before, we append the resulting measurements to
 our `m` container and we print it.
 
+Exercise: here we're not using the OONI data format and we're
+instead printing the internally used data structures. Can
+you modify the code to emit data using OONI's data format here?
+(Hint: there are conversion functions in `measurex`.)
+
 ```Go
 	print(m)
 }