docs(cost): add GCP billing setup guide for mc-cost-optimizer 0.6.5

[dogfootman]

Summary

Documents how to enable GCP billing collection in mc-cost-optimizer.

Changes

• docs/cost-optimizer-gcp-billing.md (new)
  • Prerequisites
    • Standard usage cost export to BigQuery, and table naming (gcp_billing_export_v1_<BILLING_ACCOUNT_ID>)
    • Admin SA registered at OpenBao secret/csp/gcp, with its required roles
  • /api/gcp/setup flow: status → auto-provision → dataset → billing-confirmed → restart the collector
  • Verification (log lines, manual /admin/billing/collect, SQL), troubleshooting, and the legacy (≤0.6.0) env method
• conf/docker/.env.setup
  • Added a ## Cost-GCP-Collector section with comments
  • Removed MC_COST_OPTIMIZER_GCP_BQ_DATASET / MC_COST_OPTIMIZER_GCP_BQ_TABLE, which 0.6.5 ignores (dataset/table now come from OpenBao secret/cost/gcp or auto-discovery)
• conf/docker/docker-compose.yaml: removed the matching gcp.dataset / gcp.table env lines from mc-cost-optimizer-gcp-collector
• README.md: linked the guide and added the Azure/NCP/GCP collector ports (18093/18094/28095) to the firewall table

Verification

Ran on a local stack with mc-costopti-api and mc-costopti-gcpcollector at 0.6.5:

• Before setup: the collector fell back to csp/gcp and auto-discovered the billing table. With Standard and Detailed exports in the same dataset, it picked the Detailed table. Manual collection: COMPLETED.
• Setup flow:
  • auto-provision first failed at SA key generation with 403 iam.serviceAccountKeys.create.
  • After granting the admin SA roles/iam.serviceAccountKeyAdmin, all steps returned OK.
  • dataset (existing) → billing-confirmed → stage COMPLETE.
• After setup: the collector read the new values only after a restart. The log then showed cost/gcp (운영 SA), and manual collection completed.
• Pinning the Standard table: worked with bao kv patch secret/cost/gcp table=... followed by a collector restart. BE /status showed the new table only after a BE restart.
• docker compose config -q passes with .env.setup. README links resolve.