Improve instrumentation in storage.

Also, fix some other minor bugs.

Change-Id: If72f1c058b0f47d3e378fdf80228d7e9a8db06c7
Merge branch 'exp/storage-ng' of github.com:prometheus/prometheus into exp/storage-ng
Improve storage instrumentation even more.

Add gauge for chunks and chunkdescs in memory (backed by a global
variable to be used later not only for instrumentation but also for
memory management).

Refactored instrumentation code once more (instrumentation.go is back :).

Change-Id: Ife39947e22a48cac4982db7369c231947f446e17