Dashboards: equations lose the unit of their fields ($, duration)

What happens

On a spans dashboard, a plain aggregate of a currency field shows as money. An equation on the same field shows as a plain number.

Widget query	Shows
sum(gen_ai.cost.total_tokens)	$13.82
equation|sum(gen_ai.cost.total_tokens) / count_unique(gen_ai.conversation.id)	0.152

The events API returns meta.fields = "number" for every equation|... column, so the widget cannot know that the result is money.

"Cost per run" and "cost per pull request" are the numbers people want most from AI agent traces, and both are equations.

Expected

An equation keeps the unit of its field when the other side has no unit:

• currency ÷ or × a count, count_unique, or a constant → currency
• duration ÷ or × a count or a constant → duration
• currency ± currency → currency; duration ± duration → duration

Other mixes (for example currency ÷ currency) stay a plain number.

Where it happens

• Backend: get_unit_and_type() in src/sentry/api/bases/organization_events.py handles only rate units inside equations (added in #108043). All other equations return "number".
• Frontend: getFieldRendererBase() in static/app/utils/discover/fieldRenderers.tsx uses a typed renderer for an equation only when the equation text matches a special field exactly.

Steps to reproduce

1. Send spans with gen_ai.usage.input_tokens, so that Sentry adds gen_ai.cost.total_tokens.
2. Make a big number widget with sum(gen_ai.cost.total_tokens). It shows $.
3. Make a big number widget with equation|sum(gen_ai.cost.total_tokens) / count_unique(gen_ai.conversation.id). It shows no $.

Related

• Tables show currency with 2 decimals, so avg(gen_ai.cost.total_tokens) (about $0.002 per model call) shows as <$0.01. Chart axes show the full value.
• #112628 (alerts on equations), #68167 (equations in big number widgets).