[Fleet] support setting id in new agent policy API

[juliaElastic]

Summary

Support an optional id in create agent policy API

Came up during this discussion: #121628 (comment)

To verify:

1. Send a new agent policy request with id, verify the response contains the sent id:

POST http://elastic:changeme@localhost:5620/api/fleet/agent_policies
kbn-xsrf: kibana

{"id": "my-agent-policy", "namespace":"default", "name":"My agent policy"}

2. Send a new agent policy request without id, verify it succeeds (no regression)

POST http://elastic:changeme@localhost:5620/api/fleet/agent_policies
kbn-xsrf: kibana

{"namespace":"default", "name":"My agent policy 2"}

3. Send a new agent policy request with id and has_fleet_server, verify that the response contains the sent id:

POST http://elastic:changeme@localhost:5620/api/fleet/agent_policies
kbn-xsrf: kibana

{"id": "my-fleet-server-policy", "namespace":"default", "name":"My fleet server policy", "has_fleet_server": true}

Checklist

• Unit or functional tests were updated or added to match the most common scenarios

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[nchaulet]

it works as expected 🚀

It would be nice to add an explicit integration test about this field here https://github.com/elastic/kibana/blob/main/x-pack/test/fleet_api_integration/apis/agent_policy/agent_policy.ts/#L22
also creating two time the policy with the same id is well handling 409 on the second all but It could be nice to add an integration test for that.

[kibana-ci]

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Test Failures

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Metrics [docs]

Public APIs missing comments

Total count of every public API that lacks a comment. Target amount is 0. Run node scripts/build_api_docs --plugin [yourplugin] --stats comments for more detailed information.

id	before	after	diff
fleet	1142	1143	+1

Unknown metric groups

API count

id	before	after	diff
fleet	1258	1259	+1

History

• 💚 Build #21077 succeeded 3ccf2f4

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

cc @juliaElastic