Add functionality to create links on spans

[pyohannes]

According to the specification, we must provide a way to record links on spans.

[lalitb]

#342 and #366 fixes it.

[jlisee]

@lalitb #342 was closed not merged so there is no public AddLink function in the tracer::Span API is that intentional or was it an oversight?

[owent]

@lalitb #342 was closed not merged so there is no public AddLink function in the tracer::Span API is that intentional or was it an oversight?

According to https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/api.md#specifying-links, links can be added when creating a Span. This is already supported and the codes are here https://github.com/open-telemetry/opentelemetry-cpp/blob/main/api/include/opentelemetry/trace/tracer.h#L40

[jlisee]

This is already supported and the codes are here https://github.com/open-telemetry/opentelemetry-cpp/blob/main/api/include/opentelemetry/trace/tracer.h#L40

Thanks for the hint, is this API usage right below? Where {} is filling in empty attribute maps that are required? I found I hint in the test: https://github.com/open-telemetry/opentelemetry-cpp/blob/main/api/test/trace/noop_test.cc#L54-L56 but it would be great to have this in the getting started section in the docs explicitly:

// Get context from span to link
const trace_api::SpanContext ctx = otherSpan.GetContext()

// Create link to that span (is there a typedef/alias I can use here?)
std::vector<std::pair<trace_api::SpanContext, std::map<std::string, std::string>>> links = {
    {ctx, {}}
};
      
// Create span with that link (annoying to pass an empty attribute map here)
auto s1 = tracer->StartSpan("span_with_link", {}, links);

I did like the previous API which required fewer types and place holders and more obviousness:

// Get context from span to link
const trace_api::SpanContext ctx = otherSpan.GetContext()

// Create span
auto s1 = tracer->StartSpan("span_with_link");

// Add Link
s1->AddLink(ctx, {})