Application Insights logging configuration via Microsoft.AspNetCore.ApplicationInsights.HostingStartup

[DamianEdwards]

When Application Insights is enabled in an app via Microsoft.AspNetCore.ApplicationInsights.HostingStartup it needs to enable the AppInsights logger provider by default with the following categories & levels:

Category	Level
Default	Information
System.*	Warning
Microsoft.*	Warning

Additionally, it needs to add a JSON file configuration provider pointing to an (optional) file called ApplicationInsights.settings.json which overrides the above defaults when present. This file will be in the layout of an Azure App Service app in [HomeFolder]\site\diagnostics (see here for another example). This should "just work" based on the recently added configuration driven logging settings support we added.

E.g.: D:\home\site\diagnostics\ApplicationInsights.settings.json

{
  "Logging": {
    "ApplicationInsights": {
      "Microsoft.AspNetCore.Kestrel": "Debug",
      "Microsoft.AspNetCore.Mvc": "Information"
    }
  }
}

Note about Visual Studio Debugging

When running in Visual Studio 2017 under the debugger, the existing Application Insights logger behavior needs to be preserved, i.e. all logger messages are sent to Application Insights if no instrumentation key is configured, such that they show up in the in-VS Application Insights search experience.

@glennc

[pakrym]

Current problems:

• If user adds own AI logger and want to change it's settings via appsettings.json he will not be able to because IHostingStartup adds it's configurations sources after appsettings.json is added.
• If user does not use our default web host builder and does not wire Logging section of global IConfiguration to LoggerFactory he would get everything sent to AI lightup logger because there is no default level in logging.