Add option to include ElapsedMs in the HttpContext prior to invoking EnrichDiagnosticContext

[jmenziessmith]

#389

Adds AddElapsedToHttpContext to RequestLoggingOptions. When set to true, will add the elapsedMs value to the HttpContext Items collection prior to invoking EnrichDiagnosticContext

I'm suggesting this approach in order to avoid any breaking changes to EnrichDiagnosticContext

[nblumhardt]

Thanks for the PR, and sorry about the slow reply @jmenziessmith - took some time off over the Christmas/New Year period :-)

I think this solution might be a good workaround if you're able to add a Stopwatch instance to HttpContext.Items via custom middleware. I think API discoverability is enough of an issue that it probably isn't the ideal approach for the Serilog middleware to implement directly.

I can see it would have been nice to consider this requirement when designing the Serilog middleware originally; unfortunately, it might be too late to add it in a discoverable-but-nonbreaking manner.

Assuming Activity.Current is non-null in the enricher callback, the elapsed time of the activity would also indirectly reveal the elapsed time of the web request - this could be a good middle-ground to explore, let me know if you need more info!