Enable APM tracing on Fleet managed agent

Enroll agent with default config
Agent config will be overwritten by the Fleet Agent policy
Add APM config to the installed agent config file e.g. on Mac /Library/Elastic/Agent/elastic-agent.yml

agent.monitoring:
  traces: true
  apm:
    hosts:
      - <apm host url>
    environment: <apm environment>
    secret_token: <secret token>

Restart agent to pick up the changes (on standalone agent there is hot reload): https://www.elastic.co/guide/en/fleet/current/start-stop-elastic-agent.html
APM config should be applied, and traces should be sent to the APM server configured
Observe "APM instrumentation enabled" in the agent logs

More info in agent doc: https://github.com/elastic/elastic-agent/blob/main/docs/tracing.md

Cloud configuration

ECS uses the same methodology to add APM config on production builds (e.g. BC).
Cloud logic that adds APM config: https://github.com/elastic/cloud/blob/master/scala-services/runner/src/main/scala/no/found/runner/allocation/stateless/ApmDockerContainer.scala#L434
APM is not enabled by default on deployments with SNAPSHOT builds, see all conditions here: https://github.com/elastic/cloud/pull/124414