Add unit and metric_type meta to system fields.yml

[exekias]

What does this PR do?

This change updates fields.yml defintions to include units and metric
type information

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all datasets collect metrics or logs.

How to test this PR locally

Once elastic/kibana#82273 is in, system package templates should include unit and metric_type _meta fields.

Related issues

• Relates Add support for _meta in fields.yml package-spec#42

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[elasticmachine]

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Build stats

• Build Cause: [Pull request #402 updated]

• Start Time: 2020-11-23T15:50:21.138+0000

• Duration: 8 min 16 sec

Test stats 🧪

Test	Results
Failed	0
Passed	12
Skipped	0
Total	12

[ruflin]

Nice! Already lots of place where this is used.

[fearful-symmetry]

Super happy to see us finally adding this.

[jsoriano]

Nice!

[jsoriano]

I think this should be actually a keyword 🙂

I don't think we need to handle it as a metric in any case.

[exekias]

good point, will change

[jsoriano]

unit and format seem a bit redundant most of the times, maybe one can be derived from the other. Or maybe we could add additional checks, e.g. you probably always want to format as percent a metric with percent unit, same with bytes.

[exekias]

This sounds reasonable, I would prefer to keep them separated for now, as they end up in different places (template vs index pattern), I agree we could do better at linting.

[exekias]

Thanks for the reviews! @jsoriano this is ready for another go 😇