[7.x] Add support for Composite spans in the intake API (backport #5661)

[mergify]

This is an automatic backport of pull request #5661 done by Mergify.
Cherry-pick of 93332d2 has failed:

On branch mergify/bp/7.x/pr-5661
Your branch is ahead of 'origin/7.x' by 1 commit.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit 93332d26.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   apmpackage/apm/data_stream/traces/fields/fields.yml
	modified:   apmpackage/apm/docs/README.md
	modified:   docs/fields.asciidoc
	modified:   model/span/_meta/fields.yml

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   include/fields.go

Cherry-pick of 3429db6 has failed:

On branch mergify/bp/7.x/pr-5661
Your branch is ahead of 'origin/7.x' by 4 commits.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit 3429db69.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   changelogs/head.asciidoc

no changes added to commit (use "git add" and/or "git commit -a")

Cherry-pick of b827b2b has failed:

On branch mergify/bp/7.x/pr-5661
Your branch is ahead of 'origin/7.x' by 5 commits.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit b827b2b6.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   apmpackage/apm/data_stream/traces/fields/fields.yml
	modified:   apmpackage/apm/docs/README.md
	modified:   docs/fields.asciidoc
	modified:   model/span/_meta/fields.yml

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   include/fields.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[apmmachine]

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• Start Time: 2021-07-21T15:48:26.646+0000

• Duration: 43 min 3 sec

• Commit: 91b75e7

Test stats 🧪

Test	Results
Failed	0
Passed	4541
Skipped	15
Total	4556

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