[Metrics UI] Group-by dropdown allows duplicate entries for custom fields

[sorantis]

Kibana version: master

Describe the bug:
The group-by dropdown allows duplicate entries for custom fields.

Steps to reproduce:

1. Add agent.type from Custom Fields
2. Add host.os.name from Custom Fields
3. Search for agent.name (or anything else)
4. Click Add, notice that the dialog didn't close. Keep pressing Add multiple times.
5. Close the dialog and click the Group By dropdown again.
6. You should see a lot of duplicates for agent.name.
7. At this point you have two selected groupings and tons of duplicates. Unselect one of the applied grouping and select agent.name. Notice how all duplicates are have a check.
8. Uncheck agent.name. All duplicates should disappear.

[elasticmachine]

Pinging @elastic/logs-metrics-ui (Team:logs-metrics-ui)