[apm] Return 200 instead of 404 when agent configuration does not exist.

[simitt]

Summary

Implements #113272 to return a 200 with an empty body instead of a 404 when an agent configuration is not found.

Checklist

• The change has been tested with a current version of APM Server, and is confirmed to not introdue any apm-agent facing breaking change, therefore is backwards compatible.

Steps to reproduce:

1. Start Kibana from this branch
2. Start APM Server
3. Send request to APM Server for fetching a non-existing agent config, observe that apm-server response is the same as before the Kibana change.

curl -i -k "http://localhost:8200/config/v1/agents?service.name=xyz"

Response:

HTTP/1.1 200 OK
Access-Control-Expose-Headers: Etag
Cache-Control: max-age=30, must-revalidate
Content-Type: application/json
Etag: "-"
X-Content-Type-Options: nosniff
Date: Tue, 28 Sep 2021 14:02:18 GMT
Content-Length: 3

{}

fixes #113272

cc @elastic/apm-server

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[simitt]

/test

[sorenlouv]

@elasticmachine merge upstream

[kibanamachine]

💛 Build succeeded, but was flaky

• continuous-integration/kibana-ci/pull-request

• Commit: 30e794f

• Storybooks Preview

• Flaky suites:

  • xpack-APMCypress

Metrics [docs]

✅ unchanged

History

• 💔 Build #156113 failed 63a2997

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[simitt]

@sqren is auto-backport the right label to set for backporting this to 7.15.1 and are you ok with releasing as a patch fix or should it go to 7.16?

[sorenlouv]

@sqren is auto-backport the right label to set for backporting this to 7.15.1 and are you ok with releasing as a patch fix or should it go to 7.16?

Yes, that's the right label. And backporting to 7.15.1 sounds ok. I'm honestly not sure what the policy is on changing the API like this but I think it can be seen as a bug fix.

[kibanamachine]

The following labels were identified as gaps in your version labels and will be added automatically:

• v7.16.0

If any of these should not be on your pull request, please manually remove them.

[kibanamachine]

💚 Backport successful

Status	Branch	Result
✅	7.x
✅	7.15

The backport PRs will be merged automatically after passing CI.

[kibanamachine]

The following labels were identified as gaps in your version labels and will be added automatically:

• v7.1.0

If any of these should not be on your pull request, please manually remove them.

[kibanamachine]

💔 Backport failed

Status	Branch	Result
❌	5.0	Commit could not be cherrypicked due to conflicts
❌	6.0	Commit could not be cherrypicked due to conflicts
❌	5.2	Commit could not be cherrypicked due to conflicts
❌	5.1	Commit could not be cherrypicked due to conflicts
❌	5.3	Commit could not be cherrypicked due to conflicts
❌	5.4	Commit could not be cherrypicked due to conflicts
❌	5.5	Commit could not be cherrypicked due to conflicts
❌	6.1	Commit could not be cherrypicked due to conflicts
❌	5.6	Commit could not be cherrypicked due to conflicts
❌	7.0	Commit could not be cherrypicked due to conflicts
❌	6.2	Commit could not be cherrypicked due to conflicts
❌	6.3	Commit could not be cherrypicked due to conflicts
❌	6.4	Commit could not be cherrypicked due to conflicts
❌	6.5	Commit could not be cherrypicked due to conflicts
❌	6.6	Commit could not be cherrypicked due to conflicts
❌	6.7	Commit could not be cherrypicked due to conflicts
❌	7.2	Commit could not be cherrypicked due to conflicts
❌	6.8	Commit could not be cherrypicked due to conflicts
❌	7.1	Commit could not be cherrypicked due to conflicts
❌	7.3	Commit could not be cherrypicked due to conflicts
❌	7.4	Commit could not be cherrypicked due to conflicts
❌	7.5	Commit could not be cherrypicked due to conflicts
✅	7.7
❌	7.6	Commit could not be cherrypicked due to conflicts
✅	7.10
✅	7.8
✅	7.9
✅	7.11
✅	7.12
✅	7.14
✅	7.13

Successful backport PRs will be merged automatically after passing CI.

To backport manually run:
node scripts/backport --pr 113273