Trace HTTP requests/errors

[carolynvs]

All HTTP requests and responses (initiated by Flurl, so new stuff only), are traced using the "Flurl.Http" source.

The default level is "Error". To enable tracing add the following to the app.config

  <system.diagnostics>
    <switches>
      <add name="Flurl.Http" value="Information" />
    </switches>
  </system.diagnostics>

or programmatically enable tracing via:

OpenStackNet.Tracing.Http.Listeners.Add(myListener);
OpenStackNet.Tracing.Http.Switch.Level = SourceLevels.All;

Fixes #533

[carolynvs]

@DonSchenck Take a look and let me know what you think. Before I release v1.5.0 I want to add some doc around this explaining how to hook into native .NET tracing with 1-2 common libraries such as nlog or log4net and how to just dump it all to the console too.

[DonSchenck]

YES!! There have been debugging/helping customers issues in the past where enabling .NET tracing was critical. Thanks for this.

[carolynvs]

@DonSchenck What I'd really like to see is a cut/paste config change we can give someone when they run into trouble, have them run their code and then send us the generated file. So that we have a well established way of collecting the trace information.

[DonSchenck]

Agreed. I've long wanted some sort of tracing or diagnostic tool that we can deploy.