[Fleet] added doc how to enable apm on fleet managed agent (elastic#1…

…82550)

Related to elastic/fleet-server#3516

Documented how to enable apm tracing on fleet managed agents.

x-pack/plugins/fleet/dev_docs/apm_tracing.md

@@ -0,0 +1,26 @@
+# Enable APM tracing on Fleet managed agent
+
+1. Enroll agent with default config
+2. Agent config will be overwritten by the Fleet Agent policy
+3. 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>
+```
+4. 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
+5. APM config should be applied, and traces should be sent to the APM server configured
+6. 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
+