https://gitlab.com/gitlab-org/gitlab/-/merge_requests/50449 refactored
`ExportCsvService` to support both issues and requirements. However, it
introduced an N+1 SQL query when loading labels of the list of issues
that significantly slowed down exporting of many issues.

We bring back the memoization for labels inside the
`Issues::ExportCsvService` to avoid this.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/321622