In T410533: Extend SessionLengthInstrumentMixin to support xLab Experiments we instrumented session length for the A/B test's treatment group only, i.e., within ReaderExperiments.
We should do the same for the control group, i.e., within WikimediaEvents. As a result, the ReaderExperiments implementation becomes redundant, since events can be fired for both groups from WikimediaEvents.
Acceptance criteria
- ReaderExperiments session length instrumentation is removed
- Session length is instrumented in WikimediaEvents for both groups