Add test checking that all fields are documented

[narph]

In the requirements we see the step Automated checks that all fields are documented and this is done automatically in the metricsets that are using an http input and in some cases manually for the other metricsets.
Would be great to reuse

beats/metricbeat/mb/testing/testdata.go

Line 303 in 142b859

func checkDocumented(t *testing.T, data []common.MapStr, omitFields []string) error {

and add it as a separate test for the other metricsets instead of writing this step manually.

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)