Define metric details outside of the data schema

We should remove the metric details like @type and the unit from the data schema and have that described outside in the descriptors like we are for other things like identity fields. This will reduce the size of the data schema and also allow us to consistently tag up fields as metrics without them conforming to the data/measure schema like data/order/xdm:priceTotal.

## What are the schemas that are affected by the issue
data/measure
all use of data/measure

## What are examples of products that are impacted by the issue
All Adobe solutions

@kstreeter & @cdegroot-adobe

Start with the data/measure schema and trace all use of data/measure across the affected schemas. Compare the existing descriptor treatment for identity fields and the data/order/xdm:priceTotal example; the work is done when metric details are consistently described outside the data schema and affected uses are updated.

Written by the indexing model from the issue text.