fix: Negative time reported for "time label applied" measurement

[smstone]

Pull Request

Address case where negative time for "time label applied" is reported for labels that have at some point been both "labeled" and "unlabeled" on a given issue (https://github.com/github/issue-metrics/issues/282)

Proposed Changes

Readiness Checklist

Author/Contributor

• [x] If documentation is needed for this change, has that been included in this pull request
• [x] run make lint and fix any issues that you have introduced
• [x] run make test and ensure you have test coverage for the lines you are introducing

Reviewer

• [ ] Label as either fix, documentation, enhancement, infrastructure, maintenance, or breaking