Add issue metrics to import/export (make it consistent with MR metrics)

On https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19097 we've added Merge Request metrics to import/export feature. The goal there is entirely stop depending on the events table to present merged and closed events data, even for imported MRs (see https://gitlab.com/gitlab-org/gitlab-ce/issues/41587).

Currently we're not exporting metrics for issues, which reside in a different table. I think we should consider also exporting these for consistency sake.