[docs] APM Agent configuration documentation

[bmorelli25]

PRs & docs:

• Kibana - docs live here
  • master - [docs][APM] Agent config docs kibana#40084
  • 7.x - [docs][APM] Agent config docs (#40084) kibana#40818
  • 7.3 - [docs][APM] Agent config docs (#40084) kibana#40817
• APM Server - docs live here
  • master - [docs] Agent configuration documentation apm-server#2366
  • 7.x - [docs][7.x] Agent configuration documentation (#2366) apm-server#2430
  • 7.3 - [docs][7.3] Agent configuration documentation (#2366) apm-server#2429

---

Agent config will take precedence over local config. Documentation for each agent should reflect that:

• Java: Add central config to config source list apm-agent-java#763
• Node.js: feat: add support for APM Agent Configuration via Kibana apm-agent-nodejs#1197
• Python: added central configuration to config precedence list apm-agent-python#548
• Ruby: https://github.com/elastic/apm-agent-ruby/pull/464/files#diff-01c1a79a5a1e1b67003a1885365f2cf5R216
• Go: docs: add mentions of central config apm-agent-go#598
• .NET: Implement remote (AKA central) configuration apm-agent-dotnet#352

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Here's some boilerplate documentation for APM Agent configuration (thanks to the Node.js team). Some of the agents use tables instead of unordered lists, so just adapt as necessary to your agent.

[float]
[[central-config]]
==== `centralConfig`

* *Type:* Boolean
* *Default:* `true`
* *Env:* `ELASTIC_APM_CENTRAL_CONFIG`

Enable {kibana-ref}/agent-configuration.html[APM Agent Configuration via Kibana].
If set to `true`, the client will poll the APM Server regularly for new agent configuration.

NOTE: This feature requires APM Server v7.3 or later and that the APM Server is configured with `kibana.enabled: true`.

---

Info:

APM meta: #4
Agents meta-issue: #76
Server issue: elastic/apm-server#2126
Kibana issue: elastic/kibana#34990 elastic/kibana#39555
Integration testing issue: elastic/apm-integration-testing#436
Optimization / caching issue: elastic/apm-server#2220

Alert users that:

• it may take some time for config changes to take effect.
• Different server versions may cause problems
• Add documentation that the user should make sure to define sane defaults in case the APM Server is offline (see Agent remote configuration #76 (comment))

[jalvz]

Side note: this introduces an apm-server.kibana setting that needs documentation as well. Is the same one as setup.kibana in other Beats, but under the apm-server namespace instead of kibana.

[bmorelli25]

All Agents, except for .NET, have an open or merged PR for these docs. .NET has an issue opened, which is linked to above. Closing this meta issue.