Fix Azure App Service App settings not being picked up

[rayrfan]

Issue

I have a Azure App Service App setting for Application Insights Instrumentation key, but it's not being picked up.

Cause

I mistakenly removed .AddEnvironmentVariables() which is important for Azure App settings, without it those settings will not be picked up, see this commit d249584#commitcomment-25810929

Fix

I updated the code in Program.cs to debug

        var configuration = new ConfigurationBuilder()
            .SetBasePath(Directory.GetCurrentDirectory())
            .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
            .AddJsonFile($"appsettings.{Environment.GetEnvironmentVariable("ASPNETCORE_ENVIRONMENT") ?? "Production"}.json", optional: true, reloadOnChange: true)
            .AddEnvironmentVariables() // <-- important
            .AddCommandLine(args)
            .Build();

        var instrumentKey = configuration.GetValue<string>("ApplicationInsights:InstrumentationKey");
        Log.Logger = new LoggerConfiguration()
           .ReadFrom.Configuration(configuration)
           .Enrich.FromLogContext()
           .WriteTo.ApplicationInsightsTraces(instrumentKey)
           .CreateLogger();

            Log.Warning("Instrumentation Key: " + instrumentKey);

Things Learned

The key in Azure App settings can be either ApplicationInsights:InstrumentationKey or ApplicationInsights__InstrumentationKey.

ASPNETCORE_ENVIRONMENT on Azure seems to be not existed, so I default in my code for it to be "Production".

The hosting environment default value is "production", see
aspnet/Hosting#863
https://github.com/aspnet/Hosting/blob/b6da89f54cff11474f17486cdc55c2f21f2bbd6b/src/Microsoft.AspNetCore.Hosting/Internal/HostingEnvironment.cs#L10

   public Startup(IHostingEnvironment env)
    {
        var builder = new ConfigurationBuilder()
            .SetBasePath(env.ContentRootPath)
            .AddJsonFile("appsettings.cs.json")
            .AddJsonFile($"appsettings.cs.{env.EnvironmentName}.json", optional: true);

Therefore here, the default value will be production.