fix build definition and allow for the empty tracestate

Author: Sergey Kanzhelev

.vsts/ci-myget-update.yml

@@ -41,7 +41,7 @@ steps:
   displayName: test
   inputs:
     command: "test"
-    projects: "test/**/*.csproj"
+    projects: "test/**/*.Tests.csproj"
     arguments: "--configuration Release"
 
 - task: PublishTestResults@2

src/OpenCensus/Trace/Propagation/TraceContextFormat.cs

@@ -72,15 +72,32 @@ public override ISpanContext Extract<T>(T carrier, Func<T, string, IEnumerable<s
                         {
                             if (string.IsNullOrWhiteSpace(tracestate))
                             {
-                                discardTracestate = true;
-                                break;
+                                continue;
                             }
 
                             // tracestate: rojo=00-0af7651916cd43dd8448eb211c80319c-00f067aa0ba902b7-01,congo=BleGNlZWRzIHRohbCBwbGVhc3VyZS4
                             var keyStartIdx = 0;
                             var length = tracestate.Length;
                             while (keyStartIdx < length)
                             {
+                                // first skip any prefix commas and OWS
+                                var c = tracestate[keyStartIdx];
+                                while (c == ' ' || c == '\t' || c == ',')
+                                {
+                                    keyStartIdx++;
+                                    if (keyStartIdx == length)
+                                    {
+                                        break;
+                                    }
+
+                                    c = tracestate[keyStartIdx];
+                                }
+
+                                if (keyStartIdx == length)
+                                {
+                                    break;
+                                }
+
                                 var keyEndIdx = tracestate.IndexOf("=", keyStartIdx);
 
                                 if (keyEndIdx == -1)