List observed metrics

Metric names and label values actually recorded for this org in the recent past. Use it to populate a metric picker: GET /orgs/{org_id}/metrics requires a name, and alarms reference names that only exist once something has emitted them.

origin separates the two kinds: system metrics (app.*, deployment.*, org.*) are emitted by the platform and have no configuration, while custom ones come from a metric definition — see definition_ids for the config behind them.

This reports what is flowing, not what is configured, so the two can disagree in both directions: a definition that has never matched a span will not appear, and a deleted definition's series stays until its bins age out.

Needs analytics.read for the org-wide catalog. Passing project_id narrows it to series carrying that project's dimension and accepts projects.alarms.read there instead, so a project-scoped role gets a working metric picker. That view omits series with no project dimension, org.credits.* among them — which are org-level metrics that caller cannot alarm on anyway.