Create individual fields for string metadata types

Instead of lumping them into a repeated field of string metadata, we
want to have a separate field for each string type (hostname, arch,
etc). This allows us to selectively omit fields for privacy reasons.

BUG=chromium:365023
TEST=perf_serializer_test passes

Change-Id: Ib0ae22da12de1c9ccc6d1bac7f63a29e89cb6700
Signed-off-by: Simon Que <sque@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/195686
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Emma Rapati <rapati@chromium.org>
220 files changed