Define metric details outside of the data schema

[cluby-adobe]

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

[cdegroot-adobe]

@cluby-adobe @kstreeter we need someone to do a PR on this asap to review for 0.9.9.9 release.