How does dbt Labs position the metrics layer as an independent entity between business intelligence and data transformation?

We don't need another layer. In fact, that's the last thing we need. I think adding metrics to the transformation layer is very natural. Doing dbt transformations and separately doing your LookML metric definitions in two different tools is a lot of work. It'd be far easier to have one centralized place alongside your transformations where you write metrics once and never have to think about it again.