Deprecate and Remove UOM

As a developer, I'd like to simplify the data model by unifying UOM and metric ID, so that I can simplify configuration.

Currently we have two different identifiers for metrics - metric_id and UOM, and we're sometimes using them interchangeably, it's quite confusing!

Given UOM is far more common in our tables, it makes sense to use the values of UOM as the basis, and look to retire the values of `metric_id`. However, `metric_id` is more in line with the desired state of the data model. To this effect, the plan is to make data updates so that live data has UOM = metricId (changing metricId to its equivalent UOM), and then to start refactoring references to metricId, and then finally to drop UOM from our data models.