Add SpanExporter Interface

[mayurkale22]

This PR contains an interface (SpanExporter) that allows different tracing services to export recorded data for sampled spans in their own format.

[rochdev]

When is this called? We need spans to be grouped by trace, so unless this is called at the end of every trace it wouldn't work for us.

[mayurkale22]

I was planning to add a SpanEventListener interface to record start and end of the spans. Exporters would use these methods to collect and store spans locally and export method would run every few secs to export to backend.

interface SpanEventListener {
    onStartSpan(span: Span): void;
    onEndSpan(span: Span): void;
}

interface SpanExporter extends SpanEventListener {}

Let me know if this makes sense, otherwise please suggest an alternatives.

[rochdev]

I'm not sure I understand how this addresses the issue. What we need is to be able to report traces to our backend as traces and not as individual spans. In OpenCensus for example, it's traces that were passed to this method and not individual spans.

[draffensperger]

I assume by traces here you mean all of the spans for a given trace that were generated by the process? (Since presumably the full distributed trace is being written by different microservices). Could we implement that behavior by having some kind of buffering adapter that listens for spans and groups them by trace ID and then flushes them once a certain period of time goes by? Or how might the system know that no more spans for a given trace ID within the process will be created?

[rochdev]

I assume by traces here you mean all of the spans for a given trace that were generated by the process?

Yes, but given the recent discussions it seems that exporters will be responsible for handling the batching, in which case my initial concern is solved.

[draffensperger]

Cool, yeah, I could imagine some batching utilities that exporters could use for common batching strategies, one of which could be grouping by trace ID

[rochdev]

I think this need will naturally appear if code starts to be duplicated. In the meantime, I wouldn't try to predict how batching will be done. In our case, we batch in very specific ways to optimize calls as much as possible, so we wouldn't be able to use a generic batch utility.

[mayurkale22]

Closed via #177