Add custom instrumentation through a span

[spawnia]

Still unfinished, please let me know if this is something that you would like to add before i continue work.

[DiederikvandenB]

Is this to enable tracing through Sentry?

[spawnia]

Yeah. I have been trying to replace the fetch breadcrumbs, but found that I was lacking the tracing information.

You can see how the fetch breadcrumb references the span ID.

[DiederikvandenB]

Looks like a good addition to this package, yeah!