Add failing test for StartRootSpan

[alanwest]

Relates to #2803

Just adding a test for now for discussion and to determine whether there's something we can do at the moment.

It appears the StartActivity method starts a new span parented from Activity.Current even when you pass in a new ParentContext. I imagine this might be by design since Activity.Current is tracked via AsyncLocal and maybe StartActivity doesn't want to clobber that state.

opentelemetry-dotnet/src/OpenTelemetry.Api/Trace/Tracer.cs

Lines 79 to 82 in 6b7f2dd

	public TelemetrySpan StartRootSpan(string name, SpanKind kind = SpanKind.Internal, SpanAttributes initialAttributes = null, IEnumerable<Link> links = null, DateTimeOffset startTime = default)
	{
	return this.StartSpanHelper(false, name, kind, default, initialAttributes, links, startTime);
	}

opentelemetry-dotnet/src/OpenTelemetry.Api/Trace/Tracer.cs

Lines 167 to 195 in 6b7f2dd

	private TelemetrySpan StartSpanHelper(bool isActiveSpan, string name, SpanKind kind, in SpanContext parentContext = default, SpanAttributes initialAttributes = null, IEnumerable<Link> links = null, DateTimeOffset startTime = default)
	{
	if (!this.ActivitySource.HasListeners())
	{
	return TelemetrySpan.NoopInstance;
	}
	var activityKind = ConvertToActivityKind(kind);
	var activityLinks = links?.Select(l => l.ActivityLink);
	Activity previousActivity = null;
	if (!isActiveSpan)
	{
	previousActivity = Activity.Current;
	}
	var activity = this.ActivitySource.StartActivity(name, activityKind, parentContext.ActivityContext, initialAttributes?.Attributes ?? null, activityLinks, startTime);
	if (activity == null)
	{
	return TelemetrySpan.NoopInstance;
	}
	if (!isActiveSpan)
	{
	Activity.Current = previousActivity;
	}
	return new TelemetrySpan(activity);
	}

In a scenario where a thread hop has occurred - as in this test - maybe it would be more intuitive to wipe out Activity.Current... though, not sure there's a way for us to detect we're on a new thread.

---

Confirmed that StartActivity uses Activity.Current when the parent's ActivityContext == default

https://github.com/dotnet/runtime/blob/3ae87395f638a533f37b8e3385f6d3f199a72f4f/src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/ActivitySource.cs#L252-L253

[codecov]

Codecov Report

Merging #2878 (8f75109) into main (7ce2e4c) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2878      +/-   ##
==========================================
+ Coverage   84.22%   84.23%   +0.01%     
==========================================
  Files         250      250              
  Lines        8883     8883              
==========================================
+ Hits         7482     7483       +1     
+ Misses       1401     1400       -1     

Impacted Files	Coverage Δ
...Telemetry/Internal/SelfDiagnosticsEventListener.cs	97.65% <0.00%> (+0.78%)	⬆️