title: Configure HttpClientFactory to use data from the current request context
body: <p>With the new <a href="https://www.stevejgordon.co.uk/introduction-to-httpclientfactory-aspnetcore" rel="noreferrer">HttpClientFactory</a> in ASP.NET Core 2.1, it's quite easy to configure custom HTTP clients with things like base urls, default headers etc.</p>
  
  <p>However, I haven't found  a way to centralize configuration that lets me inject headers from the current request context. For example, consider a service called with an <code>Authorization</code> header, which I wish to pass on to all the underlying services as well. It would be awesome to be able to configure this in the <code>.AddHttpClient()</code> call on <code>services</code> in the <code>Startup</code> class, but I can't figure out how to get at the request context from there.</p>
  
  <p>Any ideas?</p>