Cherry-pick #21992 to 7.x: Fix diskio and memory bugs under windows

[fearful-symmetry]

Cherry-pick of PR #21992 to 7.x branch. Original message:

What does this PR do?

Fixes some Windows bugs found under testing. We need to only call CalcIOStatistics under linux, and memory.share should be removed outside of agent, since it's a "null zero" metric under windows.

Checklist

• My code follows the style guidelines of this project
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

• pull down and build on Windows
• On windows, make sure system/diskio works, and system.process.memory.share does not work.

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[elasticmachine]

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Build stats

• Build Cause: [Pull request #21999 opened]

• Start Time: 2020-10-19T20:29:23.595+0000

• Duration: 102 min 52 sec

Test stats 🧪

Test	Results
Failed	0
Passed	2244
Skipped	506
Total	2750