# 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](038-duration-as-derived-param-kind.md) and
[039](039-metric-param-query-resolves-declared-params.md) build on this.