Collect RabbitMQ message headers

[drewhammond]

Is your feature request related to a problem? Please describe.

We store valuable identifiers in RabbitMQ (AMQP 0-9-1) message headers. In Elastic APM, these headers are collected as attributes in spans, but in OpenTelemetry they are not.

Describe the solution you'd like

We'd like the ability to toggle the collection of these message headers so that they can be included in spans. As a nice-to-have, we'd like the list of collected headers to be configurable.

Describe alternatives you've considered
We have not found an attractive alternative with only using automatic instrumentation.

[mateuszrzeszutek]

Looks really similar to: #5238
I guess it could be implemented as a generic messaging headers-extracting-thing (perhaps similar to how we're extracting HTTP headers)