Return a span from getMetrics

Change-Id: I41f339dfa1a49f07ea5ab42e30d1a1e20af97b30
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/207308
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
7 files changed