Support for Service Maps and OpenRracing

[astorm]

While elastic-apm-node-opentracing can support distributing tracing, it looks like we do not support service maps. Quoting @JiaLiPassion

And I checked the source code of elastic-apm-node apm-agent-nodejs/http-shared.js at master · elastic/apm-agent-nodejs · GitHub , to display the link of the Service Map correctly, the destionationContext and the httpContext need to be set to the Span (There is no documentation about this part, so I am not sure how to set this destination context correctly to do my own instrument). And this destination context can be only set to the Span object, and a Span need to have an active transaction, so I need to create a Transaction and a Span to display the link. So I am using some hacking way to make the link being displayed.

The linked thread has some information on manually hacking in a transaction (or moving to OpenTelemetry) but if anyone from the community has a more elegant "span only" solution we'd welcome a discussion or PR/PRs on what it would take to make this happen