The word metric is for measurement, not specifically length. I always assumed the meter comes from the metric system, not vice versa. Yes, it's the modern International System, or SI, also, but it's not wrong to call it the metric system. EVERYONE knows what it means.
When we talk about the metrics of a baseball pitcher, or anything, we aren't referring to measurements of length usually.
English Oxford Dictionary.Definition of metric - relating to or based on the metre as a unit of length, relating to or denoting a metric.