breaking: move maxmind config to config.providers

- moved maxmind to separate module
- code refactored
- simplified test

internal/maxmind/city_cache.go

@@ -0,0 +1,36 @@
+package maxmind
+
+import (
+	"github.com/puzpuzpuz/xsync/v3"
+)
+
+var cityCache = xsync.NewMapOf[string, *City]()
+
+func (cfg *MaxMind) lookupCity(ip *IPInfo) (*City, bool) {
+	if ip.City != nil {
+		return ip.City, true
+	}
+
+	if cfg.db.Reader == nil {
+		return nil, false
+	}
+
+	city, ok := cityCache.Load(ip.Str)
+	if ok {
+		ip.City = city
+		return city, true
+	}
+
+	cfg.db.RLock()
+	defer cfg.db.RUnlock()
+
+	city = new(City)
+	err := cfg.db.Lookup(ip.IP, city)
+	if err != nil {
+		return nil, false
+	}
+
+	cityCache.Store(ip.Str, city)
+	ip.City = city
+	return city, true
+}