Add tickets to improve metric abstractions

2026-09-05 22:20:03 +01:00
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+# 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](037-unify-param-declarations.md).