Set g.co/agent span attribute for Stackdriver exporter based library/version of node

[draffensperger]

Currently when we convert from a data.TraceData structure to go.opencensus.io/trace.Span structures to send to actual exports in exporterwrapper.go, none of the properties from Node are used. See the Node proto for the agent.

At least for the Stackdriver exporter, we should follow the practice of the other Stackdriver exporters and write the g.co/agent attribute based on the Node.LibraryInfo that has the language and version info for the library that wrote the spans to the agent. That would allow users to understand
what library produced the spans in a particular trace in case it needs to be upgraded.

[flands]

@draffensperger what is the latest on this?

[draffensperger]

I have some draft code for this but need to brush it off and get it out. I'll plan to work on it over the next week or so. I am aiming to get this into both the OpenCensus and OpenTelemetry services.