[BUGFIX] Avoid GetDatacenter* methods to flood Consul servers logs

When calling `GetDatacentersByDistance()` or `GetDatacentersMap()`, an
incorrect condition was used to diplay log message, thus flooding
Consul's logs.

Example of message:

```
  [WARN] agent.router: Non-server in server-only area: non_server=myClientNode area=lan
```

This message is only valid for WAN areas, filter to avoid creating
hundreds of logs/s on our clusters, each time someone is calling this
method.

Our logs were flooded by such messages when migrating our Consul servers
from 1.7.7 to 1.8.4.

This will issue fix #8663

agent/router/router.go

@@ -536,10 +536,13 @@ func (r *Router) GetDatacentersByDistance() ([]string, error) {
 		for _, m := range info.cluster.Members() {
 			ok, parts := metadata.IsConsulServer(m)
 			if !ok {
+				if areaID != types.AreaLAN {
 					r.logger.Warn("Non-server in server-only area",
 						"non_server", m.Name,
 						"area", areaID,
+						"func", "GetDatacentersByDistance",
 					)
+				}
 				continue
 			}
 
@@ -547,6 +550,7 @@ func (r *Router) GetDatacentersByDistance() ([]string, error) {
 				r.logger.Debug("server in area left, skipping",
 					"server", m.Name,
 					"area", areaID,
+					"func", "GetDatacentersByDistance",
 				)
 				continue
 			}
@@ -607,10 +611,13 @@ func (r *Router) GetDatacenterMaps() ([]structs.DatacenterMap, error) {
 		for _, m := range info.cluster.Members() {
 			ok, parts := metadata.IsConsulServer(m)
 			if !ok {
+				if areaID != types.AreaLAN {
 					r.logger.Warn("Non-server in server-only area",
 						"non_server", m.Name,
 						"area", areaID,
+						"func", "GetDatacenterMaps",
 					)
+				}
 				continue
 			}
 
@@ -618,6 +625,7 @@ func (r *Router) GetDatacenterMaps() ([]structs.DatacenterMap, error) {
 				r.logger.Debug("server in area left, skipping",
 					"server", m.Name,
 					"area", areaID,
+					"func", "GetDatacenterMaps",
 				)
 				continue
 			}