Proposal to revise use of layer metadata in USD

Hi all,
I have a “meta proposal” to update USD object modeling guidelines and seed several other, more specific proposals to follow. It addresses problems people have encountered with how USD encodes important scene “metrics” information (e.g. metersPerUnit, upAxis, …) by proposing a change from layer metadata to prim applied schemas, with rationale.

Here is the PR - feedback preferred in the PR, but OK here, too - thanks!