KAFKA-16524: Metrics for KIP-853

[kevin-wu24]

This PR implements the following gauges in KafkaRaftMetrics, BrokerServerMetrics, and ControllerServerMetrics:

KafkaRaftMetrics:

• number-of-voters: number of voters for a KRaft topic partition. All of the replicas will report this metric.
• number-of-observers: number of observers being tracked by the KRaft topic partition leader.
• uncommitted-voter-change: 1 if there is a voter change that has not been committed, 0 otherwise.

number-of-observers and uncommitted-voter-change will only be present on the active controller, since it does not make sense for other replicas to report these metrics. If number-of-offline-voters is implemented, it will also only be reported by the leader.

BrokerServerMetrics:

• ignored-static-voters: 1 if reading the voter set from the log, 0 if the voter set comes from static configuration.

ControllerServerMetrics:

• IgnoredStaticVoters: 1 if reading the voter set from the log, 0 if the voter set comes from static configuration.
• IsObserver: 1 if the controller node is not a voter for the KRaft cluster metadata partition, 0 otherwise.

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)