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deposit monitoring: check metadata

the Icinga plugin (https://forge.softwareheritage.org/source/swh-icinga-plugins/browse/master/swh/icinga_plugins/deposit.py) should also check the metadata is correct.

We just had a bug on staging that caused the metadata to be split in REMD items of one character each, and it was only noticed by HAL's manual tests.

Migrated from T3985 (view on Phabricator)