[Metrics] Metrics Explorer does not show all terms in "graph per"

[bvader]

@sorantis
@simianhacker

Kibana version: 7.6.2, 7.7 Snaphot

Elasticsearch version: 7.6.2, 7.7 Snaphot

Describe the bug:
Metrics Explorer does not show all terms in "graph per" selector after a metric has been added to the graph. All terms are show before adding metrics, and terms should show after adding a metric.

Steps to reproduce:

1. Add some additional terms to metricbeat agent / index
   example

processors:
  - add_fields:
      target: project
      fields:
        name: myproject
        id: '574734885120952459'

2. Setup and run metricbeat

3. Navigate to Metrics Explorer and start typing in field you want to "graph per" example
   Example ... project.name. Observe project.name can be selected... add a metric examplesystem.load.1and you can graph.

4. Remove metric and project.name in "graph per"

5. This time add metric first example system.load.1 and now navigate to "graph per" and try to select project.name. Observe it is not available, it appears only ECS fields are available.

Expected behavior:
I should be able to graph by arbitrary terms when added after the metric. If you remove the metric the project.name fields is available. If you add the term ahead you can graph per just fine.

Any additional context:
Screen shot from the telegraf project. tag.host is a term so there is a work around to add the term first but it does not allow easy exploration by term.

[simianhacker]

@bvader I just created a PR to remove the remaining field filtering, it will be backported to 7.7.1.