Add filter helper method

(cherry picked from commit 9b1b7d3132)

filter.go

@@ -0,0 +1,12 @@
+package dynamiclistener
+
+func OnlyAllow(str string) func(...string) []string {
+	return func(s2 ...string) []string {
+		for _, s := range s2 {
+			if s == str {
+				return []string{s}
+			}
+		}
+		return nil
+	}
+}