Add content_metadata_{add,get}.
Partially replaces !418 (closed) (part 3/3)
Depends on !436 (closed).
Migrated from D3357 (view on Phabricator)
Partially replaces !418 (closed) (part 3/3)
Depends on !436 (closed).
Migrated from D3357 (view on Phabricator)