[Metricbeat] allow empty region/zone config param for storage metricset

[kaiyan-sheng]

What does this PR do?

This PR is to allow storage metricset with no region/zone config parameter because based on the documentation: If no region is specified, it will return metrics from all buckets.
https://github.com/elastic/beats/blob/master/x-pack/metricbeat/module/googlecloud/storage/_meta/docs.asciidoc

Why is it important?

GCP storage has two different classes, regional and multi-regional. When user wants to get storage metrics using Metricbeat for regional storage resources, a region/zone name can be specified in config. But if a region/zone is specified, no metrics can be queried from multi-regional storage resources.

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 google cloud platform account has multiple storage accounts: both regional and multi-regional ones.
2. Enable googlecloud module and modify googlecloud.yml to use:

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

3. Start metricbeat and you should see metrics collected from both multi-regional and regional storage resources.

Related issues

Closes #17623

Screenshots

Here is a screenshot for showing metrics collected from all regions with config:

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

[elasticmachine]

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

[blakerouse]

Looks good.