Using the local APM-server I found out that de agent connect over both http and grpc to the apm-server as shown in the logs
{"log.level":"info","@timestamp":"2022-06-15T11:24:56.203Z","log.logger":"request","log.origin":{"file.name":"middleware/log_middleware.go","file.line":63},"message":"request ok","service.name":"apm-server","url.original":"/","http.request.method":"POST","user_agent.original":"","source.address":"xxxx","http.request.id":"9a24c6ca-0db5-4409-8892-977a5ddccc14","event.duration":45491,"http.response.status_code":200,"ecs.version":"1.6.0"}
{"log.level":"info","@timestamp":"2022-06-15T11:36:19.808Z","log.logger":"beater.grpc","log.origin":{"file.name":"interceptors/logging.go","file.line":65},"message":"","service.name":"apm-server","source.address":"xxxxxx","grpc.request.method":"/opentelemetry.proto.collector.trace.v1.TraceService/Export","event.duration":153781,"grpc.response.status_code":"OK","ecs.version":"1.6.0"}
However only the events send using grpc show up in kibana, I suspect that I need to provide a endpoint other than root / in the url but I'm not sure what is has to be.
Knowing I had a working agent I went back to connecting to Elastic Cloud and ended up deleting the port from the url which fixed the connection issue.
So I can now actually see the trace information from the OpenTelemetry agent in kibana.
@GregKalapos thank you for the helpful tips.