internal/clientinfo: use ptr cache when listing clients

internal/clientinfo/ptr_lookup.go

@@ -40,6 +40,13 @@ func (p *ptrDiscover) String() string {
 	return "ptr"
 }
 
+func (p *ptrDiscover) lookupHostnameFromCache(ip string) string {
+	if val, ok := p.hostname.Load(ip); ok {
+		return val.(string)
+	}
+	return ""
+}
+
 func (p *ptrDiscover) lookupHostname(ip string) string {
 	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
 	defer cancel()