ottyl/tickets/037-unify-param-declarations.md

037 — Unify param declarations: one Param type, referenced by name from metrics

Summary

"Parameter" currently names two structurally near-identical but separate types: ParamDeclaration (src/data_value.rs, driven by the Params panel, used only to add a display column to the logs table) and RegexToMetricPrimitiveMatcher (src/dto.rs), embedded inline inside a metric declaration. A metric cannot reference an already-declared param by name — src/engine.rs says so explicitly ("referencing an already-declared ParamDeclaration by name isn't supported yet") — so the same regex/capture-group/kind gets redefined from scratch per metric, and nothing keeps a param declared in the Params panel consistent with a same-named matcher inside a metric.

Scope

• Collapse ParamDeclaration and RegexToMetricPrimitiveMatcher into one ParamDeclaration type: { name, extractor (regex+capture_group | json_field_path), kind }.
• Change MetricDeclaration to reference params by name instead of embedding its own extractor: label_params: Vec<String>, value_param: Option<String>. Role (label vs. value) becomes a property of how a metric uses a param, not a property of the param itself.
• Update apply_metric_to_log (src/engine.rs) to resolve label_params/ value_param against the shared param declaration store instead of reading an inline matcher.
• Update declare/validate paths (validate_metric_request, engine.rs) to reject a metric referencing an undeclared param name.
• Update frontend types (webapp/src/lib/api.ts) and Metrics.tsx's declaration form to pick existing declared params (e.g. a select populated from the Params panel) rather than re-entering a regex/capture-group/kind per metric.
• Migration: existing stored MetricDeclaration records with inline matchers need their matchers turned into standalone ParamDeclaration entries (created implicitly, named e.g. <metric_name>.<param_name> if no matching declared param exists) so existing profiles keep working after upgrade.

Acceptance criteria

• A param declared once in the Params panel can be selected as a label or value param by multiple metric declarations without re-entering its extractor.
• Declaring a metric that references a nonexistent param name is rejected at declare-time with a clear error.
• Existing profiles with metrics declared under the old inline-matcher shape load and behave identically after migration.
• Metrics.tsx no longer has a separate "regex + capture group + kind" entry form duplicated from the Params panel's form.

Depends on

None. 038 and 039 build on this.