Fixes #52648

The http.server.request.duration counter records information about every HTTP request. One piece of recorded information is the http.route which is the matched route pattern. The route pattern is taken from HttpContext's Endpoint.

There is a bug that causes http.route to not be available when:

• Incoming request to an app that has enabled metrics
• There is an unhandled exception
• App is using UseExceptionHandler
• Response hasn't started or aborted
• The exception handler clears the HttpContext and re-executes the middleware pipeline <- problem
• Value is recorded to http.server.request.duration without http.route

The exception handler clears the endpoint from the context and re-executes the middleware pipeline. There is no longer an endpoint on the context when http.server.request.duration is recorded and the http.route value is lost.

The fix is to stash the original endpoint to a known location (a stable key in HttpContext.Items) and then falls back to getting the endpoint from that location if the endpoint isn't set. HttpContext.Items is used because IExceptionHandlerPathFeature (which has an Endpoint property with the original endpoint) isn't available at the hosting layer.

A magic string in a dictionary isn't ideal, but we do use this technique in some other places. It would be better to add a new feature - IOriginalEndpointFeature perhaps? - but it doesn't seem worth it for this scenario.