Fix journal client's statsd probing

recent change broke the statsd reporting at swh.journal.client level by missing the namespace argname while instanciating the Statsd class, thus making "swh_journal_client" the host os the Statsd instance.

Add a test for the usage of statsd in the journal client.

added Mirror label

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merged

mentioned in commit swh-core@218eec69

mentioned in merge request swh-core!345 (merged)