TelemetryHttpModule add url.path to tags

[Wraith2]

Added the "url.path" tag to the tags for newly created activities in StartAspNetActivity.

To ignore a path like a healthcheck it is best to use a sampler to ensure that the activity and all child activities are dropped in a sampler. To do this the sampler needs to be able to provide enough information for the sampler author to detect the path. This is only currently possible by using HttpContext.Current or other method of injecting the request into the sampler. This is unsatisfying because the caller of the sampler has information about the current request it just doesn't have a way to pass that information to the sampler.

This PR changes the way StartAspNetActivity works by adding the request.Unvalidated.Path of the request to the tags that are used to construct the activity. The tags are one of the few things that are passed to samplers when an activity is started. With this change it is now possible to write a sampler which uses only provided context information in the sampler to decide whether a particular request root activity should be dropped.

There are no public api changes.