allow cancelling MoveNext operations

src/csharp/Grpc.Core/IAsyncStreamReader.cs

@@ -41,6 +41,11 @@ namespace Grpc.Core
     /// (<c>MoveNext</c> will return <c>false</c>) and the <c>CancellationToken</c>
     /// associated with the call will be cancelled to signal the failure.
     /// </para>
+    /// <para>
+    /// <c>MoveNext()</c> operations can be cancelled via a cancellation token. Cancelling
+    /// an individual read operation has the same effect as cancelling the entire call
+    /// (which will also result in the read operation returning prematurely).
+    /// </para>
     /// </summary>
     /// <typeparam name="T">The message type.</typeparam>
     public interface IAsyncStreamReader<T> : IAsyncEnumerator<T>

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

@@ -49,10 +49,9 @@ namespace Grpc.Core.Internal
 
         public async Task<bool> MoveNext(CancellationToken token)
         {
-            if (token != CancellationToken.None)
+            var cancellationTokenRegistration = token.CanBeCanceled ? token.Register(() => call.Cancel()) : (IDisposable) null;
+            using (cancellationTokenRegistration)
             {
-                throw new InvalidOperationException("Cancellation of individual reads is not supported.");
-            }
                 var result = await call.ReadMessageAsync().ConfigureAwait(false);
                 this.current = result;
 
@@ -63,6 +62,7 @@ namespace Grpc.Core.Internal
                 }
                 return true;
             }
+        }
 
         public void Dispose()
         {

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

@@ -49,14 +49,15 @@ namespace Grpc.Core.Internal
 
         public async Task<bool> MoveNext(CancellationToken token)
         {
-            if (token != CancellationToken.None)
+            
+            var cancellationTokenRegistration = token.CanBeCanceled ? token.Register(() => call.Cancel()) : (IDisposable) null;
+            using (cancellationTokenRegistration)
             {
-                throw new InvalidOperationException("Cancellation of individual reads is not supported.");
-            }
                 var result = await call.ReadMessageAsync().ConfigureAwait(false);
                 this.current = result;
                 return result != null;
             }
+        }
 
         public void Dispose()
         {