Add CORS configuration for OpenTelemetry HTTP endpoints

[axw]

To enable OpenTelemetry-instrumented browser applications to send data to APM Server, we should add CORS, like we support for the Elastic APM RUM JavaScript agent.

We should consider replacing apm-server.rum.allow_{origins,headers} with something more general, like apm-server.cors.allow_{origins,headers}, and have it apply to all HTTP endpoints.

[PaleHazy]

Hello, I am using open telemetry today and I'm getting Cors errors trying to connect to APM server. I wonder if I'm doing something wrong. Or do I need to add some configuration?

[axw]

This has yet to be prioritised or implemented, so it's currently not possible I'm afraid.

[ejbeaty]

This has yet to be prioritised or implemented, so it's currently not possible I'm afraid.

Hey @axw - just to confirm, this means it's not possible to send open telemetry data directly from the browser to an elastic.co APM server, correct?

[axw]

@ejbeaty correct, not without some kind of proxy in front