SubscribeConfiguration is not working with Azure App Configuration

## Expected Behavior

Can subscribe configuration changed. After I change configuration value, .NET app will receive changed value.

## Actual Behavior

SubscribeConfiguration throw a exception "sentinel key is not provided in metadata"

## Steps to Reproduce the Problem

Follow quickstarts [.NET configuration sdk sample](https://github.com/dapr/quickstarts/blob/master/configuration/csharp/sdk/order-processor/Program.cs)

This sample just work with Redis as Configuration store

## Version

CLI version: 1.12.0
Runtime version: 1.12.2
SDK version: 1.12.0

## Workaround

But I found a workaround, use below code:

```csharp
if (metadata == null)
metadata = new Dictionary() { ["sentinelKey"] = key };
else
metadata = new Dictionary(metadata) { ["sentinelKey"] = key };
var subscribe = await daprClient.SubscribeConfiguration(ConfigExtensions.APPCONFIG_DEFAULT, new[] { key }, metadata, app.Lifetime.ApplicationStopped);
```