[7.14](backport #26885) Add missing outputs to apm server spec file

[mergify]

This is an automatic backport of pull request #26885 done by Mergify.
Cherry-pick of cf0cd20 has failed:

On branch mergify/bp/7.14/pr-26885
Your branch is up to date with 'origin/7.14'.

You are currently cherry-picking commit cf0cd20f9.
  (fix conflicts and run "git cherry-pick --continue")
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Changes to be committed:
	modified:   x-pack/elastic-agent/CHANGELOG.next.asciidoc
	modified:   x-pack/elastic-agent/spec/apm-server.yml

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   x-pack/elastic-agent/pkg/agent/program/supported.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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• Start Time: 2021-08-09T14:19:22.713+0000

• Duration: 73 min 59 sec

• Commit: d9abdf9

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Test	Results
Failed	0
Passed	6924
Skipped	24
Total	6948

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Test	Results
Failed	0
Passed	6924
Skipped	24
Total	6948