Add EventSource/EventCounter tracing and metrics for ResponseCaching

[analogrelay]

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

• Microsoft-AspNetCore-ResponseCaching EventSource
  • CacheSets - Counts the number of created items
  • CacheRemoves - Counts the number of explicitly removed items
  • CacheHits - Counts the number of successful cache lookups
  • CacheMisses - Counts the number of failed cache lookups
  • Eviction? Where does it happen and can it be tracked here?

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

[aspnet-hello]

This issue was moved to dotnet/aspnetcore#2601