Pass existing LoggerFactory to WebHostBuilder

[cwe1ss]

Initializing the logging system should be one of the first things in an app. Now the LoggerFactory is created in the constructor of WebHostBuilder and it can be configured by calling webHostBuilder.ConfigureLogging or by configuring the logger in your Startup class.

However, there are scenarios where you want to initialize your logging system before creating a WebHostBuilder:

• your application hosts multiple WebHosts
• some stuff happens before you create the WebHost (e.g. registering with and getting the hosting url from Service Fabric)
• your app might be a GUI app that just happens to host a WebAPI for whatever reason, ...

For this reason, I suggest that it should be possible to optionally pass an existing LoggerFactory into the WebHostBuilder. This could be done by introducing a second constructor WebHostBuilder(ILoggerFactory loggerFactory) or by introducing another method .UseLoggerFactory(ILoggerFactory loggerFactory).

I would prefer the second constructor because this makes sure that there's always just one instance of the factory and you can't mix up the calls to UseLoggerFactory and ConfigureLogging.

If you agree to this, I'd be happy to create a pull request!

[cwe1ss]

Of course, we could also just use the existing constructor with an optional parameter:

public WebHostBuilder(ILoggerFactory loggerFactory = null)
{
     _hostingEnvironment = new HostingEnvironment();
     _loggerFactory = loggerFactory ?? new LoggerFactory();
     _configureServicesDelegates = new List<Action<IServiceCollection>>();
}

[cwe1ss]

Closed to continue discussion directly in the pull request. I hope that's ok - I haven't figured out yet, whether it's better to have discussions in an issue or PR.

[JunTaoLuo]

@cwe1ss Don't close the issue until you have merged the PR. We use issues to track progress. Discussions can happen in both places though it's more design oriented in issues and more implementation oriented in PRs