match delegate signature in benchmark

src/csharp/Grpc.Microbenchmarks/Utf8Encode.cs

@@ -61,11 +61,14 @@ namespace Grpc.Microbenchmarks
             var native = NativeMethods.Get();
 
             // nop the native-call via reflection
-            NativeMethods.Delegates.grpcsharp_call_send_status_from_server_delegate nop = (CallSafeHandle call, BatchContextSafeHandle ctx, StatusCode statusCode, byte[] statusMessage, UIntPtr statusMessageLen, MetadataArraySafeHandle metadataArray, int sendEmptyInitialMetadata, byte[] optionalSendBuffer, UIntPtr optionalSendBufferLen, WriteFlags writeFlags) => {
+            unsafe
+            {
+                NativeMethods.Delegates.grpcsharp_call_send_status_from_server_delegate nop = (CallSafeHandle call, BatchContextSafeHandle ctx, StatusCode statusCode, byte* statusMessage, UIntPtr statusMessageLen, MetadataArraySafeHandle metadataArray, int sendEmptyInitialMetadata, byte[] optionalSendBuffer, UIntPtr optionalSendBufferLen, WriteFlags writeFlags) => {
                     completionRegistry.Extract(ctx.Handle).OnComplete(true); // drain the dictionary as we go
                     return CallError.OK;
                 };
                 native.GetType().GetField(nameof(native.grpcsharp_call_send_status_from_server)).SetValue(native, nop);
+            }
 
             environment = GrpcEnvironment.AddRef();
             metadata = MetadataArraySafeHandle.Create(Metadata.Empty);