[meta] Elastic Agent APM Integration with Fleet [7.14]

[simitt]

This meta issue collects work required to make APM Server managed by Elastic Agent with Fleet for 7.14. This includes work for on-prem and ESS/ECE.

APM Server

• Support relevant APM Server settings in the UI [Elastic Agent] Add supported config options for APM Server under Elastic Agent #4528
• Introduce direct agent configuration Introduce direct agent configuration #5018
• Store routable URL [fleet] Add apm-server.url to the APM fleet configuration  #4895
• Fetch source maps from Fleet Server Introduce configuration for specifying service-specific source maps #5002
• [cloud] Send APM Server configurations to Kibana APM endpoint on startup Send APM Server configurations to Kibana APM endpoint on startup #5377
• [cloud] Hardcode sizing dependent settings in APM Server Hardcode sizing dependent settings in APM Server #5372
• Support customization of throughput relevant settings in APM integration https://github.com/elastic/apm-server/issues/5387
  - [ ] Preliminary work for Java agent auto-attaching [meta] [fleet] Java agent auto attaching #4830 continued in 7.15

Others

• [apm] Inject agent config directly into Fleet policies [APM] Inject agent config directly into APM Fleet policies kibana#95501
• [apm] User flow to switch from Classic APM indices to Data streams [APM] Fleet: User flow to switch from Classic APM indices to Data streams (Cloud only) kibana#100657
• [apm] Onboarding guide
  • [apm] Integrate APM Server configuration with the APM UI [APM] Link to Fleet APM Server Configuration when managed by Elastic Agent w/Fleet kibana#88659
  • [apm] Add secret_token to agent instructions in legacy Getting Started guide [APM] Fleet: Add secret_token to agent instructions in legacy Getting Started guide kibana#100661
  • [apm] Ensure "Check APM Server status" works with fleet [APM] Fleet: Ensure "Check APM Server status" works with fleet kibana#100672
• [apm] Source map upload endpoint [APM] Fleet: Introduce API for uploading source maps for RUM kibana#95393
• [apm] Add support for data streams [APM] Fleet: Add support for data streams kibana#94702
• [fleet] Support composable templates [Fleet] Support composable templates kibana#90454
• [apm] Design for policy configuration editor https://github.com/elastic/observability-design/issues/29
• [fleet] Allow to customize pipelines per data stream [Ingest Pipelines][discuss] Support custom pipelines  package-spec#129, or at a minimum provide docs on how to customize (draft docs PR [Elastic Agent] Add docs for custom ingest pipeline and template beats#23247)
• [fleet] Support custom ILM policies per data stream [ILM] Support custom ILM policies package-spec#128
• changes in ESS/ECE