Merge ticket 051: allow deleting a declared metric

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

tickets/051-delete-metrics.md

@@ -1,64 +0,0 @@
-# 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).