Add failing test for StartRootSpan (#2878)

Author: alanwest

test/OpenTelemetry.Tests/Trace/TracerTest.cs

@@ -15,7 +15,9 @@
 // </copyright>
 
 using System;
+using System.Collections.Generic;
 using System.Diagnostics;
+using System.Threading.Tasks;
 using Xunit;
 
 namespace OpenTelemetry.Trace.Tests
@@ -74,6 +76,45 @@ public void Tracer_StartRootSpan_BadArgs_NullSpanName()
             Assert.Null(span3.Activity.DisplayName);
         }
 
+        [Fact(Skip = "See https://github.com/open-telemetry/opentelemetry-dotnet/issues/2803")]
+        public async Task Tracer_StartRootSpan_StartsNewTrace()
+        {
+            var exportedItems = new List<Activity>();
+
+            using var tracer = Sdk.CreateTracerProviderBuilder()
+                .AddSource("tracername")
+                .AddInMemoryExporter(exportedItems)
+                .Build();
+
+            async Task DoSomeAsyncWork()
+            {
+                await Task.Delay(10);
+                using (tracer.GetTracer("tracername").StartRootSpan("RootSpan2"))
+                {
+                    await Task.Delay(10);
+                }
+            }
+
+            using (tracer.GetTracer("tracername").StartActiveSpan("RootSpan1"))
+            {
+                await DoSomeAsyncWork();
+            }
+
+            Assert.Equal(2, exportedItems.Count);
+
+            var rootSpan2 = exportedItems[0];
+            var rootSpan1 = exportedItems[1];
+            Assert.Equal("RootSpan2", rootSpan2.DisplayName);
+            Assert.Equal("RootSpan1", rootSpan1.DisplayName);
+            Assert.Equal(default(ActivitySpanId), rootSpan1.ParentSpanId);
+
+            // This is where this test currently fails
+            // rootSpan2 should be a root span of a new trace and not a child of rootSpan1
+            Assert.Equal(default(ActivitySpanId), rootSpan2.ParentSpanId);
+            Assert.NotEqual(rootSpan2.TraceId, rootSpan1.TraceId);
+            Assert.NotEqual(rootSpan2.ParentSpanId, rootSpan1.SpanId);
+        }
+
         [Fact]
         public void Tracer_StartSpan_BadArgs_NullSpanName()
         {