ottyl/tickets/041-fix-string-metric-coercion.md

041 — Stop silently coercing string metric values to 1.0 in sum()/avg()

Summary

coerce_to_f64 (src/engine.rs) maps every MetricPrimitive::String value to a flat 1.0. This means sum()/avg() on a string-valued metric silently becomes an occurrence count instead of erroring or behaving predictably, with no signal to the user that their aggregate doesn't mean what it looks like it means.

Scope

• At PromQL query-evaluation time, detect when the target metric's declared value kind is String and the requested aggregation is sum/avg.
• Either reject the query with a clear error ("cannot sum/avg a string-typed metric; use count() or a numeric-typed metric"), or introduce an explicit count() aggregation so occurrence-counting is something a user opts into by name rather than something sum() does silently for the wrong metric kind.
• Audit existing dashboards/panels for sum()/avg() over string-kind metrics that would now be rejected or need updating to count().

Acceptance criteria

• Querying sum()/avg() over a string-kind metric either fails with a clear, actionable error or is redirected to an explicit count() semantic — it no longer silently returns a count while looking like a sum.
• Numeric-kind metrics are unaffected.

Depends on

None.