gossip: observability for gossip latency

There are currently no metrics for Gossip latency. This is important, for example because it affects liveness -- if gossip is too slow, liveness won't be propagated fast enough and nodes will try to force re-election.

One idea is to export a latency histogram on each receiver. Every message has a sender timestamp and we could compare that with the receiver's clock to get an estimate of the latency.

Jira issue: CRDB-34799