Improve the way for combining appsettings.json with nlog.config

[snakefoot]

Right now one one have to reload NLog.config twice. When loading NLog as first thing at startup like this:

var logger = NLog.Web.NLogBuilder.ConfigureNLog("nlog.config").GetCurrentClassLogger();

Because ConfigSetting is not initially available:

var builder = new ConfigurationBuilder()
    .SetBasePath(env.ContentRootPath)
    .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true)
    .AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: true)
    .AddEnvironmentVariables();

Configuration = builder.Build();

ConfigSettingLayoutRenderer.DefaultConfiguration = Configuration;

// Loads NLog.config one more time
var logger = NLog.Web.NLogBuilder.ConfigureNLog("nlog.config").GetCurrentClassLogger();

Could be nice if we could have a more fluent interface. Maybe like this:

var config = builder.Build();
var logger = NLog.LogManager.LogFactory
   .WithConfigSettings(config)     // Assigns DefaultConfiguration
   .WithLoggingConfiguration("nlog.config")   // Register NLog.Web.AspNetCore + NLog.Extension.Logging
   .GetCurrentClassLogger();

See also NLog/NLog#3097 and see also https://github.com/NLog/NLog/wiki/Reinitialize-NLog-configuration

[snakefoot]

NLog.Extension.Logging ver. 1.6.0 inludes #344 that improves on this by adding these extensions methods to ILoggerBuilder:

public static ILoggingBuilder AddNLog(this ILoggingBuilder builder, LoggingConfiguration configuration)
public static ILoggingBuilder AddNLog(this ILoggingBuilder builder, string configFileRelativePath)

They have the special behavior, that they assign/load the NLog-config AFTER having registered ${configsetting}. This should make config-lookup during target-initialization less error-prone.

Ground work is also being prepared for creating a fluent-config-api for the NLog LogFactory. See also NLog/NLog#3686 (Making NLog.Web.NLogBuilder.ConfigureNLog obsolete)

[snakefoot]

Made a small step with #418 and NLog/NLog.Web#540 (Waiting for NLog 4.7.1)

[snakefoot]

NLog.Web.AspNetCore ver 4.9.3 has been released, and now you can do this:

var logger = NLog.LogManager.Setup().LoadConfigurationFromAppSettings().GetCurrentClassLogger();

That replaces the old style:

var logger = NLog.Web.NLogBuilder.ConfigureNLog("nlog.config").GetCurrentClassLogger();