ottyl/tickets/038-duration-as-derived-param-kind.md

038 — Make Duration an explicit derived-param kind, not a hidden case of Value

Summary

Today a Value-role param with kind: Duration silently means something different from every other kind: instead of parsing extracted text, it means "elapsed time since the previous same-labeled event" (apply_metric_to_log, src/engine.rs, backed by a LastEventTimestamp index entry). Same enum variant (MetricPrimitiveKind::Duration), two unrelated meanings depending on role — one is "parse this text as a duration," which doesn't apply here at all; the other is "compute elapsed time from event sequencing." This is a silent special case a reader has to already know about to understand the code.

Scope

• Add an explicit extractor variant, e.g. Extractor::ElapsedSinceLast { grouped_by: Vec<String> }, distinct from Regex/JsonPath.
• A param declared with this extractor is computed (elapsed time since the previous event sharing the given label set), never text-extracted. MetricPrimitiveKind no longer needs a role-dependent special case for Duration.
• Update apply_metric_to_log to branch on extractor kind instead of on (role, kind) combination.
• Update the Params panel / metric declaration UI (Params.tsx, Metrics.tsx) to offer "elapsed since previous event" as its own extractor option rather than a regex+kind combination that happens to mean something else.
• Migration: existing declared/embedded params using Value role + Duration kind convert to the new extractor variant.

Acceptance criteria

• A "duration" param's behavior (elapsed time since last matching event) is selectable directly in the UI without going through a regex field that is never actually used.
• apply_metric_to_log has no branch that changes meaning based on role for a given kind.
• Existing profiles using the old Value+Duration combination behave identically after migration.

Depends on

037.