Three built images on the cg network with config baked in: Tempo (OTLP ingest,
ports 4317/4318), Prometheus, and Grafana with both datasources auto-provisioned.
Config lives in infra/observability/. The verify-observability step runs early in
CI verify-stack; the three containers are added to the failure log-dump list.
Tempo's Grafana plugin has no datasource /health method, so the check proves
Tempo reachability through Grafana's datasource proxy instead.
refs #122
Polls Grafana's datasource-health endpoints so it proves Grafana can actually
reach Tempo and Prometheus over the cg network, not merely that the containers
started. Fails until the backplane is wired (next commit).
refs #122
not