One presumes that this is a person who compares the style of the questioned work with the style of known works of the author to reach a conclusion.

Your analysis is convincing, Father Steve.

I assume "metrician" derives from "metric" which is defined thus [Dictionary.com]:

met·ric
1. A standard of measurement.
2. Mathematics. A geometric function that describes the
distances between pairs of points in a space.

Cross-pollinating from your "humour" thread ["Blood, phlegm, choler or bile?"], I wonder if we could describe George Santayana as a humourmetrician?