From time to time people ask in foren how to extract the network utilization values of fritz 7390. Some time ago I found a page of AVM where they described in detail the algorithm how to authorize with java (The description is in German but the Java code is self explanatory).
I used the java program code to create a prototype in python. This way you can read also other webpages of fritzbox. Use and extend the code if you want. There also exists a bash prototype with curl. If you use go just visit this page.
For another nice source of information about hard- and software about AVM and Fritzbox see Hemmerling: FRITZ!Box.