[Metricbeat] googlecloud compute metricset missing exported field in doc

[kaiyan-sheng]

https://www.elastic.co/guide/en/beats/metricbeat/master/exported-fields-googlecloud.html is empty and I think this is because under https://github.com/elastic/beats/tree/master/x-pack/metricbeat/module/googlecloud/compute/_meta there is no file called data.json.

In compute metricset, we should have a compute_integration_test.go file that has TestData function to test/write events to data.json under _meta. This file will be used for exported fields in documentation.

[elasticmachine]

Pinging @elastic/integrations-platforms (Team:Platforms)

[botelastic]

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