Fix documented return type of metadata_provider_add.

Summary:
As the in-mem storage was written based on this documentation, fix it as well.

Add a test for this behavior.

Depends on D716.

Reviewers: Reviewers, ardumont

Reviewed By: Reviewers, ardumont

Subscribers: ardumont, swh-public-ci

Differential Revision: https://forge.softwareheritage.org/D717