Increase deadline precision offset

[JamesNK]

Timers and Datetime.UtcNow aren't precise. They can be wrong by up to 14ms on some OSes.

Increase deadline timer safety margin to 14ms. Prevents deadline timer being rescheduled, at the cost of a small delay in triggering deadline. Triggering deadline slightly later than the specified value is fine.

Making change because of flakey test

grpc-dotnet/test/FunctionalTests/Client/HedgingTests.cs

Lines 188 to 223 in 21a1412

	[TestCase(1)]
	[TestCase(2)]
	public async Task Unary_DeadlineExceedAfterServerCall_Failure(int exceptedServerCallCount)
	{
	var callCount = 0;
	var tcs = new TaskCompletionSource<DataMessage>(TaskCreationOptions.RunContinuationsAsynchronously);
	Task<DataMessage> UnaryFailure(DataMessage request, ServerCallContext context)
	{
	callCount++;
	if (callCount < exceptedServerCallCount)
	{
	return Task.FromException<DataMessage>(new RpcException(new Status(StatusCode.DeadlineExceeded, "")));
	}
	return tcs.Task;
	}
	// Arrange
	var method = Fixture.DynamicGrpc.AddUnaryMethod<DataMessage, DataMessage>(UnaryFailure);
	var serviceConfig = ServiceConfigHelpers.CreateHedgingServiceConfig(nonFatalStatusCodes: new List<StatusCode> { StatusCode.DeadlineExceeded });
	var channel = CreateChannel(serviceConfig: serviceConfig);
	var client = TestClientFactory.Create(channel, method);
	// Act
	var call = client.UnaryCall(new DataMessage(), new CallOptions(deadline: DateTime.UtcNow.AddMilliseconds(200)));
	// Assert
	var ex = await ExceptionAssert.ThrowsAsync<RpcException>(() => call.ResponseAsync).DefaultTimeout();
	Assert.AreEqual(StatusCode.DeadlineExceeded, ex.StatusCode);
	Assert.AreEqual(StatusCode.DeadlineExceeded, call.GetStatus().StatusCode);
	Assert.IsFalse(Logs.Any(l => l.EventId.Name == "DeadlineTimerRescheduled"));
	}