List custom metric definitions

The org's custom metric definitions and their current predicates.

project_id filters to what is in effect for that project, which includes org-wide definitions (those with no project_id) since they run against that project's runs too.

This lists configuration, not data. A definition that has never matched a span appears here but not in GET /orgs/{org_id}/metrics/catalog.

Filtered per definition the same way GET /orgs/{org_id}/alarms is: org-wide definitions need alarms.read, project ones need projects.alarms.read on the owner. So a project-scoped caller sees their own definitions but not the org-wide ones that also run against their runs, even with project_id set.