[Metricbeat] Separate vpc metricset to vpn, transitgateway and natgateway metricsets

[kaiyan-sheng]

What does this PR do?

This PR is to separate vpc metricset into three smaller metricsets: vpn, transitgateway and natgateway metricsets. This PR will also add dashboards for each metricset.

Why is it important?

This will help with users who only wants to enable one or two out of all three metricsets.

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

Screenshots

vpn dashboard:

nat gateway dashboard:

transit gateway dashboard:

How to test this PR locally

Make sure the AWS account you use for testing has metrics for VPN, NATgateway, transit gateway.
Enable aws module and then change aws.yml to:

- module: aws
  period: 1m
  credential_profile_name: elastic-test
  metricsets:
    - vpn
    - natgateway
    - transitgateway

You should see metrics collection per every minute and dashboards should start showing data points as well.

[elasticmachine]

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