title: How to log unhandled exception in .net core
body: <p>With ELMAH feature in web api 2.0 and Centralized logging and error handling , runtime calls logging module and decide if it can be handled then calls the handler else just logs it.. 
  how can this feature be added in web api core. as we don't have inbuilt centralized unhandled logging feature..</p>
  
  <p>One way I got is to use ExceptionHandler middleware and when it gets called, get the exception and log  it or send email.. but what if handler doesn't get called .. how to log those unhandled exceptions ? </p>