Cherry-pick #22845 to 7.x: Duplicate fields from system.process to process.*

[kaiyan-sheng]

Cherry-pick of PR #22845 to 7.x branch. Original message:

What does this PR do?

This PR is to rename several fields from system.process metricset to process.*. Here is the full list:

• system.process.state -> process.state
• system.process.cpu.total.norm.pct -> process.cpu.pct
• system.process.cpu.start_time -> process.cpu.start_time
• system.process.memory.rss.pct -> process.memory.pct

cc @fearful-symmetry @mukeshelastic

Why is it important?

This is a part of the inventory schema work for process. Please see #22216 for more details.

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.

[elasticmachine]

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

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: Pull request #22945 opened

• Start Time: 2020-12-05T01:16:41.737+0000

• Duration: 65 min 24 sec

Test stats 🧪

Test	Results
Failed	0
Passed	2319
Skipped	521
Total	2840

Steps errors

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Terraform Apply on x-pack/metricbeat/module/aws

• Took 0 min 14 sec . View more details on here

Terraform Apply on x-pack/metricbeat/module/aws

• Took 0 min 15 sec . View more details on here

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	2319
Skipped	521
Total	2840

[ycombinator]

Backport LGTM.