[Metricbeat] Add region to googlecloud module config

[kaiyan-sheng]

What does this PR do?

This PR is to add region as a config parameter in googlecloud module. If region is specified, instances in all zones under that region will be collected.

Why is it important?

This way, if users need to collect metrics from us-west-1a, us-west-1b and us-west-1c, they don't need to specify one section of config for each zone in googlecloud.yml. For example:

- module: googlecloud
  metricsets:
    - compute
  zone: "us-west-1a"
  project_id: elastic-observability
  credentials_file_path: "/Users/kaiyansheng/Downloads/elastic-observability-d17781618202.json"
  exclude_labels: false
  period: 300s
- module: googlecloud
  metricsets:
    - compute
  zone: "us-west-1b"
  project_id: elastic-observability
  credentials_file_path: "/Users/kaiyansheng/Downloads/elastic-observability-d17781618202.json"
  exclude_labels: false
  period: 300s
- module: googlecloud
  metricsets:
    - compute
  zone: "us-west-1c"
  project_id: elastic-observability
  credentials_file_path: "/Users/kaiyansheng/Downloads/elastic-observability-d17781618202.json"
  exclude_labels: false
  period: 300s

Instead, with this PR, only region: us-west-1 is needed in the config:

- module: googlecloud
  metricsets:
    - compute
  region: "us-west-1"
  project_id: elastic-observability
  credentials_file_path: "/Users/kaiyansheng/Downloads/elastic-observability-d17781618202.json"
  exclude_labels: false
  period: 300s

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

How to test this PR locally

Use config below, googlecloud metricset should collect compute metrics from all instances under us-central1 region, which includes us-central1-a, us-central1-b, us-central1-c, and us-central1-f.

- module: googlecloud
  metricsets:
    - compute
  region: "us-central1"
  project_id: elastic-observability
  credentials_file_path: "/Users/kaiyansheng/Downloads/elastic-observability-d17781618202.json"
  exclude_labels: false
  period: 300s

Related issues

• Closes [Metricbeat] Add regions support in googlecloud module #15780

[kaiyan-sheng]

I will revisit this PR after #15598 get merged.
Update: rebase is done.

[sayden]

Really good! I'm impressed 😃