# 051 — Allow deleting a declared metric

## Summary

There is currently no way to delete a metric once declared. `src/api.rs`
only routes `GET`/`POST /api/metrics` and `GET`/`PUT /api/metrics/{name}`
(`src/api.rs:132-135`) — there is no `DELETE`. The engine already has the
machinery for it internally: `EngineMessage::RemoveMetricDeclaration`
(`src/engine.rs:103`) removes a metric's declaration from the index, but
it is only ever sent as part of `delete_profile`'s cleanup
(`src/api.rs:689`) when an entire profile is deleted, not as a
user-facing action on a single metric. Persistence has the same gap:
`Settings`/profile storage (`src/profile.rs`, post ticket 049 now inline
in `settings.json`) has `record_metric_declaration`/
`record_metric_declaration_update` but no corresponding removal method, so
even if a metric were removed from the live engine index there'd be
nothing to stop it reappearing on next load. The webapp UI
(`webapp/src/components/Metrics.tsx`) has no delete affordance either —
compare to param declarations, which have the same underlying gap (no
`RemoveParamDeclaration`-triggering UI or persisted-removal path either),
though this ticket is scoped to metrics only.

## Scope

- Add `DELETE /api/metrics/{name}` in `src/api.rs`, sending
  `EngineMessage::RemoveMetricDeclaration` to the engine (same message
  the profile-delete cleanup path already uses) and returning 404 if the
  metric doesn't exist.
- Add a persisted-removal method (e.g. `Settings::remove_metric_declaration`
  or equivalent, alongside `record_metric_declaration`/
  `record_metric_declaration_update` in `src/profile.rs`/`src/settings.rs`)
  so a deleted metric doesn't reappear when declarations replay from
  `settings.json` on the next restart, and call it from the new route the
  same way `persist_metric_declaration` is called from `declare_metric`.
- Add a delete affordance in `webapp/src/components/Metrics.tsx` (icon
  button or similar, following whatever confirm-before-destructive-action
  pattern already exists elsewhere in the settings/profile UI — check
  `NavMenu.tsx`'s profile deletion for the existing convention) that calls
  the new endpoint and removes the metric from the list on success.
- Decide and document what happens to bucket-value history already
  recorded under the deleted metric's name (e.g. `MetricPrimitive` samples
  indexed by metric name in the engine) — either leave it orphaned
  (queryable by name won't work without a declaration, but doesn't need
  active cleanup) or purge it; state the choice in the PR.
- Decide and document what happens to dashboard panels or alert rules
  whose PromQL query text/filter references the deleted metric by name —
  at minimum they should fail gracefully (a clear "unknown metric" error
  rather than a panic or silent empty result) rather than needing to be
  actively kept in sync; a full dependent-reference check/warning before
  delete is out of scope unless trivial.

## Acceptance criteria

- A metric can be deleted via the API and the UI; it disappears from
  `GET /api/metrics` immediately and does not reappear after a restart.
- Deleting an unknown metric name returns a clear 404, not a 500 or panic.
- A dashboard panel or alert rule referencing the deleted metric by name
  fails with a readable error rather than crashing the engine or API,
  verified with a seeded fixture.

## Depends on

None (touches the same engine/persistence seam as ticket 049, but that
ticket is already merged).