add exception info to Status.Detail if metadata credentials fail

src/csharp/Grpc.Core/Internal/NativeMetadataCredentialsPlugin.cs

@@ -68,7 +68,8 @@ namespace Grpc.Core.Internal
             }
             catch (Exception e)
             {
-                Native.grpcsharp_metadata_credentials_notify_from_plugin(callbackPtr, userDataPtr, MetadataArraySafeHandle.Create(Metadata.Empty), StatusCode.Unknown, GetMetadataExceptionStatusMsg);
+                var detail = GetMetadataExceptionStatusMsg + " " + e.ToString();
+                Native.grpcsharp_metadata_credentials_notify_from_plugin(callbackPtr, userDataPtr, MetadataArraySafeHandle.Create(Metadata.Empty), StatusCode.Unknown, detail);
                 Logger.Error(e, GetMetadataExceptionLogMsg);
             }
         }
@@ -87,7 +88,8 @@ namespace Grpc.Core.Internal
             }
             catch (Exception e)
             {
-                Native.grpcsharp_metadata_credentials_notify_from_plugin(callbackPtr, userDataPtr, MetadataArraySafeHandle.Create(Metadata.Empty), StatusCode.Unknown, GetMetadataExceptionStatusMsg);
+                string detail = GetMetadataExceptionStatusMsg + " " + e.ToString();
+                Native.grpcsharp_metadata_credentials_notify_from_plugin(callbackPtr, userDataPtr, MetadataArraySafeHandle.Create(Metadata.Empty), StatusCode.Unknown, detail);
                 Logger.Error(e, GetMetadataExceptionLogMsg);
             }
         }

src/csharp/Grpc.IntegrationTesting/MetadataCredentialsTest.cs

@@ -153,9 +153,10 @@ namespace Grpc.IntegrationTesting
         [Test]
         public void MetadataCredentials_InterceptorThrows()
         {
+            var authInterceptorExceptionMessage = "Auth interceptor throws";
             var callCredentials = CallCredentials.FromInterceptor(new AsyncAuthInterceptor((context, metadata) =>
             {
-                throw new Exception("Auth interceptor throws");
+                throw new Exception(authInterceptorExceptionMessage);
             }));
             var channelCredentials = ChannelCredentials.Create(TestCredentials.CreateSslCredentials(), callCredentials);
             channel = new Channel(Host, server.Ports.Single().BoundPort, channelCredentials, options);
@@ -163,6 +164,7 @@ namespace Grpc.IntegrationTesting
 
             var ex = Assert.Throws<RpcException>(() => client.UnaryCall(new SimpleRequest { }));
             Assert.AreEqual(StatusCode.Unavailable, ex.Status.StatusCode);
+            StringAssert.Contains(authInterceptorExceptionMessage, ex.Status.Detail);
         }
 
         private class FakeTestService : TestService.TestServiceBase