Migration dashboard

Noted by @stanhu - it would be good to have a single dashboard on https://performance.gitlab.net to watch during the migration.

Placeholder issue - we need to decide what goes into it and then actually create the board.

Created dashboards

• Azure: https://performance.gitlab.net/dashboard/db/gcp-failover-azure
• GCP: https://dashboards.gitlab.net/d/YoKVGxSmk/gcp-failover-gcp

Todo

• CI

• Database writes

rate(pg_stat_database_xact_commit{datname="gitlabhq_production", environment="prd", tier="db", type="postgres"}[$__interval]) rate(pg_stat_database_xact_rollback{datname="gitlabhq_production", environment="prd", tier="db", type="postgres"}[$__interval])