this patch adds two gauges:
scylla_gossip_live - how many live nodes the gossiper sees
scylla_gossip_unreachable - how many nodes the gossiper tries to connect
to but cannot.
Both metrics are reported once per node (i.e., per node, not per shard) it
gives visibility to how a specific node sees the cluster.
For example, a split-brain 6 nodes cluster (3 and 3). Each node would
report that it sees 2 nodes, but the monitoring system would see that
there are, in fact, 6 nodes.
Example of two nodes cluster, both running:
``
scylla_gossip_live{shard="0"} 1.000000
scylla_gossip_unreachable{shard="0"} 0.000000
``
Example of two nodes cluster, one is down:
``
scylla_gossip_live{shard="0"} 0.000000
scylla_gossip_unreachable{shard="0"} 1.000000
``
Fixes#10102
Signed-off-by: Amnon Heiman <amnon@scylladb.com>
Closes#10103
[avi: remove whitespace change and correct spelling]
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