Add support for metric-storage (Aetos) service #3636

What is missing?

I'd like to add support for the Aetos service to gophercloud. Aetos is a Keystone-authenticated reverse proxy for Prometheus that enforces multi-tenant RBAC on all Prometheus queries. It is registered in the Keystone service catalog under the type metric-storage.

The service exposes the standard Prometheus HTTP API under /api/v1/ with OpenStack authentication. A Python client already exists for this service (python-observabilityclient).

It would be organized as a single package openstack/metric/v1/metrics/ covering the following endpoints:

• GET /api/v1/query — Instant query
• GET /api/v1/labels — List label names
• GET /api/v1/label/{name}/values — List label values
• GET /api/v1/series — Find series by label matchers
• GET /api/v1/targets — Target discovery overview
• GET /api/v1/status/runtimeinfo — Server runtime info
• POST /api/v1/admin/tsdb/clean_tombstones — Clean tombstones
• POST /api/v1/admin/tsdb/delete_series — Delete series
• POST /api/v1/admin/tsdb/snapshot — Create TSDB snapshot

Plus a NewMetricV1() service client constructor in openstack/client.go.