text: Title: [APM] Prefix internal APM APIs with `/internal/`
  Body: All APM apis with very few exceptions should be marked as internal. We can do that easily by prefxing a route with `/internal/`. APM apis are defined in: [`x-pack/plugins/apm/server/routes`](https://github.com/elastic/kibana/tree/9dff4d0827e8e1424ca4d86f692dec6c738f84c3/x-pack/plugins/apm/server/routes)
  
  Public APIs:
   - [Agent configuration APIs](https://www.elastic.co/guide/en/kibana/master/agent-config-api.html)
   - [Annotations APIs](https://www.elastic.co/guide/en/kibana/master/apm-annotation-api.html)
   - [Source maps](https://www.elastic.co/guide/en/kibana/master/rum-sourcemap-api.html)
  
  Additional context: https://docs.google.com/document/d/1v4R9hWiQObP8sERfflGScC9Yp36fLE0wVlUNf13mVrc/edit#heading=h.2b1v1tr0ep8m