PIM-1401 Fixed displayed conversion unit fields for a given channel

```
Bug fix: no
Feature addition: yes
Backwards compatibility break: no
Unit test passes: yes
Behat scenarios passes: ?
Checkstyle issues: no
ChangeLog updated: no
Documentation PR:
Fixes the following jira:
 - PIM-1401
```
In fact, one field should be displayed for each metric attributes (and
not one field per metric family). This allows to define two different
conversion units for two metric attributes that are using the same
metric family.