Add ILogger support for TryRefreshAsync in .NET Core package

[avanigupta]

This PR adds a new public property to the IConfigurationRefresher interface to support logging any exceptions that occur during refresh operations:
ILoggerFactory IConfigurationRefresher.LoggerFactory { get; set; }

Using ILoggerFactory with Dependency Injection

If you want to get your ILoggerFactory through dependency injection, the default ILoggerFactory will be added automatically when services.AddAzureAppConfiguration() is invoked in your ConfigureServices method. No code changes are needed.

Using ILoggerFactory without Dependency Injection

If you don't use dependency injection, you can set your own instance of ILoggerFactory on the IConfigurationRefresher instance.

IConfigurationRefresher refresher = null;
var config = new ConfigurationBuilder()
            .AddAzureAppConfiguration(options =>
            {
                options.ConfigureRefresh(refreshOptions =>
                {
                    refreshOptions.Register("Sentinel", true)
                        .SetCacheExpiration(TimeSpan.FromSeconds(30));
                })

                refresher = options.GetRefresher();
            })
            .Build();

refresher.LoggerFactory = _loggerFactory;
bool success = await refresher.TryRefreshAsync();

Logging Category
The category of refresh logs will be: Microsoft.Extensions.Configuration.AzureAppConfiguration.Refresh

Breaking Change
This is a breaking change because we added a new dependency on Microsoft.Extensions.Logging package and the public interface has changed.

The AspNetCore middleware doesn't need any special changes to support ILogger.

[jimmyca15]

Instead of having ILogger as an argument of TryRefresh, can we have ILogger registered on our configuration provider?

[avanigupta]

Instead of having ILogger as an argument of TryRefresh, can we have ILogger registered on our configuration provider?

Our configuration provider is created during app startup but dependency injection takes place after startup has completed. So AzureAppConfigurationProvider will not be able to get ILogger from DI. Or maybe I'm missing something? What is the benefit of registering ILogger with config provider instead of as an argument of TryRefreshAsync?

[jimmyca15]

It is a bit of a chicken and egg problem in the context of asp.net core dependency injection. Passing ILogger to each invocation of TryRefreshAsync just seems a bit odd to me. I expect ILogger to be more of a state of the refresher object than an argument to operations on the refresher.

Having a settable logger property on IConfigurationRefresher would seem more reasonable to me, but having it on the provider itself sounds even better.

[jimmyca15]

Your PR description is great! The one thing I would add is to the part about dependency injection to call out that the logger factory will be wired up automatically as long as services.AddAzureAppConfiguration is called.

Also, the category that we log with.