KIP-1063 — Drop inconsistent casing in Selector metrics tags

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Standardizes metric tag name casing across Kafka's internal metrics to lower camelCase (e.g., broker-id → brokerId, BrokerId → brokerId, clientId already correct), resolving inconsistent formats across ControllerChannelManager, BrokerBlockingSender, and AbstractFetcherManager. Inconsistent tag casing complicates metric queries and dashboards that rely on exact tag name matching.

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