add bracket format (#15)

Reviewed-on: #15
Co-authored-by: andrey <andrey@4devs.io>
Co-committed-by: andrey <andrey@4devs.io>

field/field.go

@@ -501,3 +501,14 @@ type Field struct {
 func (f Field) String() string {
 	return fmt.Sprintf("%s=%+v", f.Key, f.Value)
 }
+
+// String implent stringer.
+func (f Field) IsKey(keys ...string) bool {
+	for _, key := range keys {
+		if key == f.Key {
+			return true
+		}
+	}
+
+	return false
+}