[Metricbeat GCP] Storage metricset requires region as input parameter

[kaiyan-sheng]

When testing storage metricset with config file:

- module: googlecloud
  metricsets:
    - storage
  project_id: elastic-observability
  credentials_file_path: "/Users/kaiyansheng/Downloads/elastic-observability-d17781618202.json"
  exclude_labels: false
  period: 300s

I got an error message:

kaiyansheng@KaiyanMacBookPro:~/Documents/test-7.7.0-bc5/metricbeat-7.7.0-darwin-x86_64$ ./metricbeat -e
...
2020-04-08T14:57:58.909-0600	INFO	[monitoring]	log/log.go:154	Uptime: 92.690166ms
2020-04-08T14:57:58.909-0600	INFO	[monitoring]	log/log.go:131	Stopping metrics logging.
2020-04-08T14:57:58.909-0600	INFO	instance/beat.go:444	metricbeat stopped.
2020-04-08T14:57:58.909-0600	ERROR	instance/beat.go:932	Exiting: 1 error: region and zone in Google Cloud config file cannot both be empty accessing config
Exiting: 1 error: region and zone in Google Cloud config file cannot both be empty accessing config

This conflicts with storage metricset documentation: If no region is specified, it will return metrics from all buckets. in https://github.com/elastic/beats/blob/master/x-pack/metricbeat/module/googlecloud/storage/_meta/docs.asciidoc

[elasticmachine]

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