To provide a better feedback about what was imported and make it easier
to debug missing objects, not the FinishImportWorker will log the number
of objects imported, like:

```
{
  "message": "GitHub project import finished",
  "duration_s": 377.68,
  "objects_imported": {
    "github_importer_imported_pull_requests_merged_by": 92,
    "github_importer_imported_pull_request_reviews": 81,
    "github_importer_imported_diff_notes": 93,
    "github_importer_imported_pull_requests": 108,
    "github_importer_imported_notes": 794,
    "github_importer_imported_issues": 321
  },
  "import_source": "github",
  "project_id": 38,
  "import_stage": "Gitlab::GithubImport::Stage::FinishImportWorker"
}
```

Changelog: changed