Display routes for owin request metrics rather than the URL path

Currently a request timer metric is created for every route of a request e.g. /sample/1, /sample/2. This commit allows a callback function to resolve the metric name using for example web api's ApiExplorer to name the metrics as the route instead e.g /sample/{id}.

Owin.Metrics is still independent of any web framework, so resolving the route template for the OWIN request will have to be implemented for the chosen web framework. The sample project has this implemented for WebApi on top of OWIN.

If you happy will this pull request could I get a new nuget package pushed please.