Add IsZero to CPU Totals, report value for monitoring

[fearful-symmetry]

What does this PR do?

This fixes three issues:

• Gives CPUTotals an IsZero method, so we don't report {} fields during certain events
• Fills out the Value field for GetSelf(), which is used by beats self-monitoring, and apparently that value is used by the elastic_agent dashboard.
• Make processes fetched with GetSelf() persist across calls

[elasticmachine]

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• Start Time: 2022-07-12T17:28:30.500+0000

• Duration: 10 min 26 sec

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Total	173

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[fearful-symmetry]

Okay, tinkered with this a bit more. The logic that adds value the way it does is ancient, and I don't fully understand why it's like that, but for now I've just decided to add the value metric in a way that...seems more reasonable?

[cmacknz]

1. Add a changelog entry for this and create a release+tag for this change.
2. As a follow up create a beats PR updating this package, and then backport it to the 8.3 branch. There is going to be an 8.3.3 release this fix can go out in.