Add telemetry to retrieve the value ExP returns

[luabud]

We already have the query-expfeature event, which includes the abexp.queriedfeature property. This helps us track which variables are being invoked in code. If we also included something like abexp.featurevalue (similar to the treatmentvalue property in the tasClientReadTreatmentComplete event from VS Code core), it would give us better visibility into whether experiments are retrieving the expected value from ExP