storage/app: add additional metadata keys
storage/app: add additional metadata keys

This also moves the metadata generation into processUpload; it didn't
really benefit from being in a separate function.

Change-Id: Ia9fb3455cf8851ea89171bc7e135d69e6734eafc
Reviewed-on: https://go-review.googlesource.com/35210
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

golang/perf,golang/perf