Ability to create a root span (span with no parent) when another span is active.

[lalitb]

As discussed here - #2410, creating the issue to evaluate/discuss support for creating a root span when another span is active.

{
    auto root1 = tracer->StartSpan("span 1");
    auto root2 = trace->StartSpan("span 2");

    auto token = tracer->WithActiveSpan(root1); // root 1 active
    auto child_root1 = tracer->StartSpan("child of root1"); // child of root1

    trace_api::StartSpanOptions options;
    options.parent = root2;
    auto child_root2 = tracer->StartSpan("child of root2", options); // child of root2
  
    // trying to create a span with no parent now.
    options.parent = SpanContext(false, false);
    auto root3 = tracer->StartSpan("root3", options); // root3 is child of root1 which is not desired
   
    opentelemetry::context::Context c1;
    options.parent = c1;
    root3 = tracer->StartSpan("root3", options); // root3 is child of root1 which is not desired

    trace_api::StartSpanOptions options2;
    root3 = tracer->StartSpan("root3", options2); // root3 is child of root1 which is not desired

}

When one can create a span that is a child to a non-active span, should it also be possible to create a span with no parent?

[alevenberg]

cc @dbolduc

[ThomsonTan]

@lalitb will propose 2 options, 1 breaks ABI and the other doesn't