Reset column information before running CommonMetricsImporter

What does this MR do?

This resets the column information before the importing the metrics. This makes sure the newly added columns are available for the migration.

What are the relevant issue numbers?

Original migrations introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21459

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/51305

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides