Fix flaky UtcNow_IsMonotonic test

Author: NachoEchevarria

tracer/test/Datadog.Trace.Tests/TraceContextTests.cs

@@ -40,6 +40,7 @@ public void UtcNow_IsMonotonic()
             var traceContext = new TraceContext(_tracerMock.Object);
 
             var t1 = traceContext.Clock.UtcNow;
+            System.Threading.Thread.Sleep(1); // Getting some flaky errors in .NET 10 (same time is returned)
             var t2 = traceContext.Clock.UtcNow;
             var substractResult = t2.Subtract(t1);