metadata: Avoid the use of raw wr_str or write_all.

They are, with a conjunction of `start_tag` and `end_tag`, commonly
used to write a document with a binary data of known size. However
the use of `start_tag` makes the length always 4 bytes long, which
is almost not optimal (requiring the relaxation step to remedy).
Directly using `wr_tagged_*` methods is better for both readability
and resulting metadata size.