[metal] Add label support

Bug: skia:13446
Change-Id: If1cb6a27858cfcee10439c70cdac2e66761e05a3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/557392
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
8 files changed