[7.x](backport #28461) Add diagnostics collect command to elastic-agent.

[mergify]

This is an automatic backport of pull request #28461 done by Mergify.

---

Mergify commands and options

More conditions and actions can be found in the documentation.

You can also trigger Mergify actions by commenting on this pull request:

• @Mergifyio refresh will re-evaluate the rules
• @Mergifyio rebase will rebase this PR on its base branch
• @Mergifyio update will merge the base branch into this PR
• @Mergifyio backport <destination> will backport this PR on <destination> branch

Additionally, on Mergify dashboard you can:

• look at your merge queues
• generate the Mergify configuration with the config editor.

Finally, you can contact us on https://mergify.io/

[botelastic]

This pull request doesn't have a Team:<team> label.

[elasticmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2021-10-19T15:32:31.498+0000

• Duration: 98 min 34 sec

• Commit: b37de0c

Test stats 🧪

Test	Results
Failed	0
Passed	7069
Skipped	24
Total	7093

💚 Flaky test report

Tests succeeded.

🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• /package : Generate the packages and run the E2E tests.

• /beats-tester : Run the installation tests with beats-tester.

[michel-laterman]

/package

[michel-laterman]

a test in e2e-x-pack/elastic-agent-packaging-linux failed when installing a .deb when it wasn't able to install it from a URL

[2021-10-19T21:15:08.691Z] time="2021-10-19T21:15:07Z" level=error msg="Error executing command" args="[exec -u root -i fleet_elastic-agent_1 apt install /filebeat-7.16.0-SNAPSHOT-arm64.deb -y]" baseDir=. command=docker env="map[]" error="exit status 100" stderr="\nWARNING: apt does not have a stable CLI interface. Use with caution in scripts.\n\nE: Sub-process Popen returned an error code (2)\nE: Encountered a section with no Package: header\nE: Problem with MergeList /filebeat-7.16.0-SNAPSHOT-arm64.deb\nE: The package lists or status file could not be parsed or opened.\n"

however the same test succeeded in the E2E Tests