Metric version of "footage"?

The answer is in your question: 'length'. No value is added by introducing the unit of measurement into the concept. But in the UK we inconsistently still talk about 'mileage' driven in our cars whilst measuring our carpets in metres.

You can't use "meterage" as this has a different meaning, the act of measuring. See Colins Dictionary. So I think you would have to record it as "The length in meters" to avoid possible confusion in situations where both imperial and metric units might be used. It's far safer to do so as there have been several recorded accidents, failures and near accidents where one unit was interpreted as the other.
There is no metric equivalent to the word footage that I, as an engineer and native BrE speaker, have ever heard.