get rid of compiler warnings

src/csharp/Grpc.Auth/GoogleAuthInterceptors.cs

@@ -33,6 +33,7 @@
 
 using System;
 using System.Threading;
+using System.Threading.Tasks;
 
 using Google.Apis.Auth.OAuth2;
 using Grpc.Core;
@@ -72,9 +73,10 @@ namespace Grpc.Auth
         public static AsyncAuthInterceptor FromAccessToken(string accessToken)
         {
             GrpcPreconditions.CheckNotNull(accessToken);
-            return new AsyncAuthInterceptor(async (context, metadata) =>
+            return new AsyncAuthInterceptor((context, metadata) =>
             {
                 metadata.Add(CreateBearerTokenHeader(accessToken));
+                return Task.FromResult<object>(null);
             });
         }
 

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

@@ -78,7 +78,10 @@ namespace Grpc.Core.Internal
             {
                 var context = new AuthInterceptorContext(Marshal.PtrToStringAnsi(serviceUrlPtr),
                                                          Marshal.PtrToStringAnsi(methodNamePtr));
-                StartGetMetadata(context, callbackPtr, userDataPtr);
+                // Don't await, we are in a native callback and need to return.
+                #pragma warning disable 4014
+                GetMetadataAsync(context, callbackPtr, userDataPtr);
+                #pragma warning restore 4014
             }
             catch (Exception e)
             {
@@ -87,7 +90,7 @@ namespace Grpc.Core.Internal
             }
         }
 
-        private async Task StartGetMetadata(AuthInterceptorContext context, IntPtr callbackPtr, IntPtr userDataPtr)
+        private async Task GetMetadataAsync(AuthInterceptorContext context, IntPtr callbackPtr, IntPtr userDataPtr)
         {
             try
             {