Metrics: add last acked heartbeat timestamp for each follower/learner

[drmingdrmer]

I guess this won't work in the case where no changes will be made to the state machine after a follower node becomes offline as the applied log indexes will remain the same, thus the difference will not change.

You're correct. To better assess the connectivity status of a follower node, it would be beneficial to add a metric that tracks the timestamp of the last acknowledged heartbeat from each follower. This additional information would provide a more accurate and timely indication of the follower's connectivity status. Here's how we could refine this idea:

1. Add a new field to the RaftMetrics struct specifically for follower heartbeat information:

pub struct RaftMetrics<C: RaftTypeConfig> {
    // ... existing fields ...
    pub follower_heartbeats: HashMap<C::NodeId, Instant>,
}

2. Update this field whenever a follower acknowledges a heartbeat or successfully replicates an entry:

impl<C: RaftTypeConfig> RaftMetrics<C> {
    pub fn update_follower_heartbeat(&mut self, follower_id: C::NodeId) {
        self.follower_heartbeats.insert(follower_id, Instant::now());
    }
}

3. In the leader's routine that checks follower health:

const HEARTBEAT_TIMEOUT: Duration = Duration::from_secs(/* define your timeout */);

for (follower_id, last_heartbeat) in &raft_metrics.follower_heartbeats {
    if last_heartbeat.elapsed() > HEARTBEAT_TIMEOUT {
        // Consider this follower as potentially disconnected
    }
}

Originally posted by @drmingdrmer in https://github.com/datafuselabs/openraft/discussions/1174#discussioncomment-10079123

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[SteveLauC]

This issue can be closed as it was completed in #1177.