Add EventSource/EventCounter tracing and metrics for Session

_From @anurse on Wednesday, November 1, 2017 3:22:31 PM_

Some events listed below. The goal is to add events that will help users solve problems so if these aren't going to do that job, feel free to suggest/discuss others!

* `Microsoft-AspNetCore-Session` EventSource
* `SessionsCreated` - Counts sessions created
* `SessionsLoaded` - Counts the number of times a session is loaded from the provider (this happens on each request for a session)
* `SessionsCommitted` - Counts the number of times a session is commited to the provider (again, happens on each request)
* We can't track abandoned or timed-out sessions any more as they are handled by caching now and we don't provide a way to explicitly abandon a session. We could detect "empty" sessions (with no data) but such a session could always be added to again later.

See https://gist.github.com/anurse/af1859663ac91c6cf69c820cebe92303 for some guidance on adding EventSources and EventCounters to ASP.NET projects.

_Copied from original issue: aspnet/Session#200_