kvserver: improve raft transport metrics

We have metrics for the send queue size and whether we're dropping based on the queue size, but we could add additional metrics for the send loop, which only has a "message sent" counter so far:

https://github.com/cockroachdb/cockroach/blob/449f0979bfe993137b206691ce8d81e7969a43ae/pkg/kv/kvserver/raft_transport.go#L689-L726

Concretely, the bytes sent (labeled by target node) and potentially even a histogram of payload sizes, though that is likely overkill.

xref https://github.com/cockroachlabs/support/issues/3342

Jira issue: CRDB-51871