Duplicate fields from system.process to process.*

[kaiyan-sheng]

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.

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

[elasticmachine]

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Build stats

• Build Cause: Pull request #22845 updated

• Start Time: 2020-12-04T22:26:55.239+0000

• Duration: 63 min 39 sec

Test stats 🧪

Test	Results
Failed	0
Passed	2320
Skipped	523
Total	2843

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Test stats 🧪

Test	Results
Failed	0
Passed	2320
Skipped	523
Total	2843

[andresrc]

should we do this in the package as well?

[kaiyan-sheng]

@andresrc Do you mean adding the new fields into the system integration? I'm planning to create a PR to move it into the integration after this PR is merged.

[fearful-symmetry]

LGTM