Add ShutdownChannelsAsync api
diff --git a/src/csharp/Grpc.Core/Channel.cs b/src/csharp/Grpc.Core/Channel.cs
index b58a6a7..e0fc718 100644
--- a/src/csharp/Grpc.Core/Channel.cs
+++ b/src/csharp/Grpc.Core/Channel.cs
@@ -88,6 +88,7 @@
                     this.handle = ChannelSafeHandle.CreateInsecure(target, nativeChannelArgs);
                 }
             }
+            GrpcEnvironment.RegisterChannel(this);
         }
 
         /// <summary>
@@ -209,6 +210,7 @@
                 GrpcPreconditions.CheckState(!shutdownRequested);
                 shutdownRequested = true;
             }
+            GrpcEnvironment.UnregisterChannel(this);
 
             shutdownTokenSource.Cancel();