I’d like to make a suggestion that the OpenType Variations spec defines a way to record whether an axis changes the font’s metrics. Of course, in principle any axis may change font metrics. But there are significant cases where an axis would not change metrics. Some that come to mind are:
1. Fonts with grades, e.g. Font Bureau Poynter
2. Fonts intended for layering and animation along axes, e.g. DJR Lab ; or an imagined version of FF Blur  with a Blur axis that offers different blur settings.
The advantage is that text does not need to be reflowed if the axis record asserts that metrics do not change. Such an assertion could be useful for users as well as text rendering engines.
For implementation in the spec, there seems to be a simple place, if the idea is a good one, that is: one of the flags in the VariationAxisRecord
for each axis, defined here:https://www.microsoft.com/typography/otspec/fvar.htm
Fonts built before this spec change would work fine, even if they could have had the flag set. Thus the flag should have 1 as the setting that asserts static metrics for the axis.