tl;dr - it's possible for addons to register and record events without riding the trains, we're doing the same for scalars; comments on the design?
A bit more
With the current Telemetry API, it's not possible to record in a scalar unless it was defined in the Scalars.yaml [1] registry. To support better support System addons and experiments, we're extending the Telemetry API to allow for dynamically registering new scalars (without touching the registry file) and record in them.
Please have a look at the attached design document [2] for comments or suggestion about the design.