Cherry-pick #20680 to 7.x: Change disk and network metrics to use Sum statistic method

[kaiyan-sheng]

Cherry-pick of PR #20680 to 7.x branch. Original message:

What does this PR do?

This PR is to change disk and network metrics from Average to Sum statistic method. This way, the values are total in the collection period, not an average. For host inventory schema work, we identified several common host metrics and for disk and network, these metrics should be gauges, which is the total value in a period of time.

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. Enable aws module and change aws.yml to:

- module: aws
  period: 5m
  credential_profile_name: elastic-beats
  metricsets:
    - ec2

2. Start Metricbeat

3. Check metrics in Kibana and compare with CloudWatch portal with Statistic=Sum and Period=5 Minutes:

[elasticmachine]

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

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #20737 opened]

• Start Time: 2020-08-21T18:01:35.196+0000

• Duration: 80 min 30 sec

Test stats 🧪

Test	Results
Failed	0
Passed	3544
Skipped	797
Total	4341

[narph]

LGTM