XRay attributes using OpenTelemetry

**Tell us about your request**
I've just instrumented my application using OpenTelemetry. Data is showing just fine in xray but I would like to add some custom fields to xray annotations. Currently I am adding custom attributes (`opentelemetry.trace.getActiveSpan()?.setAttribute('user_id', user.user_id);`) to the span and that is added to the metadata but i find no way of making that part of the annotations.

For what I can see in [the documentation](https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/7bf2266a025425993a233f66c77a0810ab11a78b/exporter/awsxrayexporter#exporter-configuration) if I want to add custom attributes to the annotations I would need to edit the collector's configuration. Is there any way of configuring this?

**Describe alternatives you've considered**
I have tried `opentelemetry.trace.getActiveSpan()?.setAttribute('user_id', user.user_id);` but it gets added into the metadata part of xray