server: Log contextual server parts when Serf node joins or leaves. (#26848)

When a Nomad server joined or left the cluster via Serf, we emit
a log message to indicate this. It was previously using a single
kv context to detail the server name, dc, and address which likely
was created before the switch to hclog and use of contextual
logging.

This change splits the log message, so that each server part is
a kv entry, which will be easier to parse with log collecting
tools.

nomad/serf.go

@@ -59,7 +59,7 @@ func (s *Server) nodeJoin(me serf.MemberEvent) {
 			s.logger.Warn("non-server in gossip pool", "member", m.Name)
 			continue
 		}
-		s.logger.Info("adding server", "server", parts)
+		s.logger.Info("adding server", "name", parts.Name, "addr", parts.Addr, "dc", parts.Datacenter)
 
 		// Check if this server is known
 		found := false
@@ -243,7 +243,7 @@ func (s *Server) nodeFailed(me serf.MemberEvent) {
 		if !ok {
 			continue
 		}
-		s.logger.Info("removing server", "server", parts)
+		s.logger.Info("removing server", "name", parts.Name, "addr", parts.Addr, "dc", parts.Datacenter)
 
 		// Remove the server if known
 		s.peerLock.Lock()