Additional metrics

Derived metrics built from two existing fields (e.g. sum(keyEvents) / sum(sessions)). They appear under Additional in every widget's metric picker and aggregate correctly (ratio of sums). Edits save to this browser; Export & commit the JSON to ship them to everyone.

Tip: long-standing ratios (Key Event Rate, Conversion Rate, ROAS, CTR, Weighted Position) are built into the widgets already; use this for metrics on top of those. Operand ids must be real, summable fields from the chosen source.