Add configuration option to extract headers from incoming requests/responses to add as span attributes

[stebet]

It'd be awesome if we could configure a list of request/response headers that should be extracted and added as span attributes.

Like what is possible here: https://open-telemetry.github.io/opentelemetry-js/interfaces/_opentelemetry_instrumentation_http.HttpInstrumentationConfig.html#headersToSpanAttributes

[evanderkoogh]

Yeah I have looked into supporting those, but there are currently more pressing issue to do in the time that I have..

What is possible though is a method called withNextSpan which takes a set of attributes that will be added to the next span.. so if you call that just before your fetch call (or any other call that is handled by a third party library), those attributes will be added to that generated span.