fix(kafka): Set namespace for Loki kafka metrics

[benclive]

What this PR does / why we need it:

Sets metric namespace to "loki" and adds the "distributor_" prefix to all the metrics generated by Kafka.

• These currently show up as "_kafka_..." and "kafka_..." in prometheus
• I considered setting the "distributor" prefix in the registerer but we have some metrics that are missing this prefix (stream_sharding_count), but have been released for a long time so it would likely break some dashboards/alerts to change it now.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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