@mulby: We can definitely set up a function for whenever a block is rendered. However, this event is particular to split tests, in that it's trying to note that a particular child of a split test has rendered, so that it's easier for researchers to identify what users have seen which splits.

Adding the usage_id to all xblock callback handlers seems out of scope for this particular PR, but is certainly doable.

We can switch to a `.` separated syntax. However, in the case of xblocks, they aren't really being generated by edx, or by the courseware. At best, I would think this would be something like `xblock.split-test.child-render`, or something. I guess I worry a little bit that trying to set up strict hierarchy is going to fail, since events may be emitted via different paths (for instance, `student_views` are rendered in the courseware, but also in studio for previews).