perf(config): optimize getfullname (#5328)

Co-authored-by: qiuwenhao <qiushaotest@qq.com>

core/conf/config.go

@@ -368,5 +368,5 @@ func getFullName(parent, child string) string {
 		return child
 	}
 
-	return strings.Join([]string{parent, child}, ".")
+	return parent + "." + child
 }

core/conf/config_test.go

@@ -1377,3 +1377,23 @@ func (m mockConfig) Validate() error {
 
 	return nil
 }
+
+func TestGetFullName(t *testing.T) {
+	tests := []struct {
+		parent string
+		child  string
+		want   string
+	}{
+		{"", "child", "child"},
+		{"parent", "child", "parent.child"},
+		{"a.b", "c", "a.b.c"},
+		{"root", "nested.field", "root.nested.field"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.parent+"."+tt.child, func(t *testing.T) {
+			got := getFullName(tt.parent, tt.child)
+			assert.Equal(t, tt.want, got)
+		})
+	}
+}