[the-glu]

This PR adds a few raft metrics:

• raft_peer_latency_seconds
• raft_peer_latency_average_seconds
• raft_peer_latency_stddev_seconds
• raft_peer_latency_min_seconds
• raft_peer_latency_max_seconds
• raft_peer_send_success_total
• raft_peer_send_fail_total

Metrics are added to a dashboard:

Tested locally on a two-node cluster.

[MariemBaccari]

I think this PR still needs to import raft.json in the dashboard and add it to Helm deployments as well.
In addition, we now have 3 different raft groups (rid, aux and scd) so it should be updated to distinguish the metrics of each group.