Add some metrics helpful for monitoring raft cluster state.
Furthermore, we weren't emitting bolt metrics on regular (non-perf) standbys, and there were other metrics
in metricsLoop that would make sense to include in OSS but weren't. We now have an active-node-only func,
emitMetricsActiveNode. This runs metricsLoop on the active node. Standbys and perf-standbys run metricsLoop
from a goroutine managed by the runStandby rungroup.
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