[Metricbeat] Add dashboard for googlecloud storage metricset

[kaiyan-sheng]

What does this PR do?

This PR is to add dashboard for googlecloud storage metricset.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

1. Make sure your GCP account for testing has storage buckets
2. Enable googlecloud module: ./metricbeat modules enable googlecloud
3. Edit modules.d/googlecloud.yml to:

- module: googlecloud
  metricsets:
    - storage
  project_id: "your project id"
  credentials_file_path: "your JSON credentials file path"
  exclude_labels: false
  period: 5m

4. Start metricbeat: ./metricbeat -e
5. Wait for 5 minutes and you should see data points in GCP storage dashboard.

Screenshots

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request [Metricbeat] Add dashboard for googlecloud storage metricset #18172 updated]

• Start Time: 2020-05-05T14:52:16.101+0000

• Duration: 94 min 13 sec (5652517)

• Commit: c4da75c

Test stats 🧪

Test	Results
Failed	0
Passed	3020
Skipped	756
Total	3776

[elasticmachine]

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

[ChrsMark]

lgtm