Add counts for registry migration skipped reasons

Similar to https://gitlab.com/gitlab-org/gitlab-exporter/-/merge_requests/166, this MR adds counts of the various reasons imports are being skipped in Phase 2 of the container registry migration. This will allow us to monitor for any potential problems as we work through all of the container repositories.

Here is a sample query and explain plan: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/9866/commands/34935