Make the return value of LoggerConfiguration.CreateLogger() concrete

[nblumhardt]

This PR communicates the dispose requirement of the logging pipeline by returning a concrete disposable type, Logger, from CreateLogger().

One consequence is that using syntax can be applied:

using (var log = new LoggerConfiguration()
    .WriteTo.Console()
    .CreateLogger())
{
    // <app runs here>
}

It's not anticipated that users will generally wrap logger creation in a using block however; rather in 2.0 the CloseAndFlush() method will cover apps that base logging off of the static Log class.

Log.Logger = new LoggerConfiguration()
    .WriteTo.Console()
    .CreateLogger();

// <app runs here>

Log.CloseAndFlush();

Doing this however clarifies at the API level that the logger needs to be flushed in some way. This is important as .NET Core doesn't expose any AppDomain unload-time hooks to trigger flushing, which historically we've relied on.

See also #718, where working around this design issue has lead us astray.

Caveats

This regresses one minor point - a logger with no sinks configured is no longer switched for a SilentLogger behind the scenes; users relying on this behaviour instead need to avoid configuring the logger at all. The alternative - returning something that could be shared in common with SilentLogger, would require one of the less-appealing options discussed in Alternatives below.

Another caveat is that in order to prevent a public-surface-area explosion, the constructors of this type are internal. This should not be a significant issue as ILogger should be the interface used in application code.

Alternatives

Instead of exposing the concrete Logger class, the following options were also available:

• Return a disposable interface - ILoggingPipeline or similar: disposable interfaces are a semantic wart since only implementations determine the need for disposal
• Return a new abstract base class - this adds the need for virtual dispatch in more places, for example level checking, which adds up if not kept under control

[merbla]

👍 I like this. It is clear where the responsibility lies. I have been caught out not closing and flushing loggers. This clears up this problem.