Add new Prometheus metrics to investigate write pressure

Part of #2142 (closed)

This change adds a few new Prometheus metrics to aid investigating whether write pressure is coming from losing writes to the cache. See this comment for more background info: #2142 (comment 243125224)

Tracking losing writes is not easy to do since we have a "last-write-wins" strategy. Typically, the winner has no extra information to determine it is the winner. However, we are able to cheat by utilizing an in-memory registry of currently in-progress entries. This will work on gitlab.com since we have a 1-to-1 Gitaly-to-Storage relationship.