odometerM

Vehicle odometer reading in metres, when one is available.

Nullable and last because most sources never have it. It exists because an odometer-primary algorithm — where the odometer is the distance of record and GPS merely corroborates — is otherwise impossible to express through this seam at all: you cannot derive an odometer reading from a stream of GPS fixes. Caught by a cross-model design review; adding it now costs nothing, adding it later would change every call site.