We can currently count the number of field hashes per form signature. Although UKM storing field hashes are only recorded at validation event. 

Ideally number of field should be recorded early on within form load ukm.

Also this need to be recorded separately at time of form submit as well to account for dynamic forms.

This will be useful in identifying thresholds for autofill. Autofill is not run on small forms. although it is unclear if this is causing autofill to not trigger for cases where it should. Having this measure will help us better tune our thresholds and potentially identify more restrictive rules regarding what autofill calls a short form.