Fix error in seeding issues

What does this MR do and why?

!121544 (merged) introduced a regression in seeding issues. Gitlab::Usage::MetricDefinition.paths is an array of paths that need to be globbed, not a an array of filenames. metric_definitions_changed? failed because it attempted to retrieve the modification time of a glob instead of a filename. To fix the error, process the path as a glob first and find the last change from all files.

Relates to gitlab-development-kit#1874 (closed)

How to set up and validate locally

1. With the latest master, run bundle exec rake db:seed_fu FILTER=09_issues.
2. It should fail at the end.
3. Repeat with this branch.

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