Task/refactor metric import

Bug fix: yes
Feature addition: no
Backwards compatibility break: no
Unit test passes: yes
Behat scenarios passes: yes
Checkstyle issues: no
ChangeLog updated: no
Documentation PR: 
Fixes the following jira:

- Refactors metric import to use property transformers
- Metric import now fully tested
- Corrected bug where metric unit was not validated in import