# 004 — Metrics API

## Summary

Expose param/metric declarations and bucket data over HTTP so the frontend (and the
filter/dashboard query engines) can declare metrics and read their values.

## Scope

- `POST /api/params` — declare a param ([001](001-param-declarations.md)).
- `POST /api/metrics` — declare a metric ([002](002-metric-declarations.md)).
- `GET /api/metrics` — list declared metrics (+ their declared params/labels).
- `GET /api/metrics/:name` — query bucket values for a metric, filtered by label
  selector and time/sequence range, mirroring the existing `LogsParams`/
  `TimelineParams` pattern in `dto.rs`.
- Follow the existing engine architecture: new `EngineQuery`/`EngineMessage` variants,
  handled in `engine.rs`'s single-owner loop, DTOs added to `dto.rs`, routes added to
  `api.rs`'s router — no new locking or shared state.

## Acceptance criteria

- Declaring a param/metric via the API round-trips through to engine state and shows
  up in `GET /api/metrics`.
- Querying a metric's bucket values supports the same start/end and limit-style
  params as the existing logs/timeline endpoints.
- Bad declarations (invalid regex etc.) return a 4xx with a useful error body, not a
  500 or silent no-op.

## Depends on

- [003 — Metric bucket derivation](003-metric-bucket-derivation.md)