feat(loki.source.kafka): Add rack_id to fetch from the closest replica

[nanwp]

Brief description of Pull Request

Expose Sarama's Config.RackID as an optional rack_id argument on loki.source.kafka. When brokers run a rack-aware replica selector, a consumer that sends a rack id fetches from the closest replica instead of the partition leader (KIP-392), which keeps traffic zone-local.

Pull Request Details

The argument is optional and defaults to an empty string, so existing configurations are unaffected. The value flows from Arguments through Convert() into kafkatarget.TargetConfig, and is applied to the Sarama client configuration.

The Sarama client configuration was built inline in NewSyncer, which also opens the broker connection, so it could not be tested. This moves that construction into newSaramaConfig. Tests now cover the rack_id wiring and the consumer group assignor mapping, which previously had none.

otelcol.receiver.kafka already exposes rack_id; this brings loki.source.kafka in line with it, and the documentation wording follows that component's page.

The promtail config converter is unchanged because promtail's Kafka target configuration has no rack setting to convert from.

Issue(s) fixed by this Pull Request

Fixes #6952

Notes to the Reviewer

PR Checklist

• Documentation added
• Tests updated
• Config converters updated
• This pull request was substantially generated with AI assistance (see the GenAI policy)

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Signed commits report

1 of 2 commits between main and feat/loki-source-kafka-rack-id could not be fully verified:

Commit	Author	Reason	Message
fd9df62b	Nanda Wijaya Putra	unknown_key	feat(loki.source.kafka): Add rack_id to fetch from the closest replica

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