Duplicated records in `issue_metrics` and `merge_request_metrics` tables.

Summary

Duplicated records in issue_metrics and merge_request_metrics tables.

Steps to reproduce

The cause is likely to be some kind of race condition. It's hard to reproduce it but happens constantly on gitlab.com (checked the DB).

What is the current bug behavior?

When creating/updating merge request or issue records, we should have always one associated metrics record.

Example query to check duplicates:

select merge_request_id from merge_request_metrics group by merge_request_id having count(*) > 1 order by merge_request_id desc limit 10;

What is the expected correct behavior?

The example query above should return 0 items.

Possible fixes

Ensure that we're not creating duplicated records (unique index). Migrate/dedup existing records