Validate span.type and .subtype against shared spec

[mikker]

Closes #1018

• Found out that we still sent types in the legacy joined form type.subtype.action. Turns out this is still Good Behaviour™
• Instead of changing any of the metadata we currently report, this PR only checks against and adds to the spec. This is because we want to get a full picture of the status quo before we possibly start aligning between agents.

[apmmachine]

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• Start Time: 2021-07-06T18:27:30.639+0000

• Duration: 32 min 34 sec

• Commit: bbdc73c

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[SylvainJuge]

Just a few minor questions/comments on my end for the shared JSON spec.

I start to feel that we could add another descriptive field to indicate which agent is using what, that will later help to align and find which ones are mostly used, and which ones are the exceptions & oddities, what do you think ?

[mikker]

I think marking agent langs in the spec is a good idea so long as we are in the process of aligning 👍

[SylvainJuge]

I've just updated the original PR (elastic/apm#443) with __used_by field, not sure if the wording/layout of this is the best but at least we can use it.

[SylvainJuge]

There are only a few differences with the elastic/apm#443 PR, thus you can directly update it once this is merged without doing a PR on PR :-).