fix: enable tempo trace metrics generator

[dearzubi]

Summary

• Enable Tempo local-blocks metrics generation so Grafana Traces Drilldown TraceQL metrics queries have a generator ring.
• Create the Tempo generator WAL and traces directories during compose startup.

Test Plan

• pnpm test
• pnpm lint
• docker compose --env-file .env -f infra/compose/docker-compose.yml config --quiet
• tempo -config.verify=true
• Verified Grafana Drilldown TraceQL metrics query returns backend series with exemplars.