add tests for foreign context propagation token

src/csharp/Grpc.Core.Tests/ContextPropagationTest.cs

@@ -154,5 +154,28 @@ namespace Grpc.Core.Tests
             await call.RequestStream.CompleteAsync();
             Assert.AreEqual("PASS", await call);
         }
+
+        [Test]
+        public void ForeignPropagationTokenInterpretedAsNull()
+        {
+            Assert.IsNull(new ForeignContextPropagationToken().AsImplOrNull());
+        }
+
+        [Test]
+        public async Task ForeignPropagationTokenIsIgnored()
+        {
+            helper.UnaryHandler = new UnaryServerMethod<string, string>((request, context) =>
+            {
+                return Task.FromResult("PASS");
+            });
+
+            var callOptions = new CallOptions(propagationToken: new ForeignContextPropagationToken());
+            await Calls.AsyncUnaryCall(helper.CreateUnaryCall(callOptions), "xyz");
+        }
+
+        // For testing, represents context propagation token that's not generated by Grpc.Core
+        private class ForeignContextPropagationToken : ContextPropagationToken
+        {
+        }
     }
 }