test(discovery): add a Configs test showing that the custom unmarshalling/marshalling is broken.

This went under the radar because the utils are never called directly.

We usually marshall/unmarshal Configs as embeded in a struct using UnmarshalYAMLWithInlineConfigs/MarshalYAMLWithInlineConfigs
which bypasses Configs' custom UnmarshalYAML/MarshalYAML

Signed-off-by: machine424 <ayoubmrini424@gmail.com>

discovery/discovery_test.go

@@ -0,0 +1,36 @@
+// Copyright 2024 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package discovery
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"gopkg.in/yaml.v2"
+)
+
+func TestConfigsCustomUnMarshalMarshal(t *testing.T) {
+	input := `static_configs:
+- targets:
+  - foo:1234
+  - bar:4321
+`
+	cfg := &Configs{}
+	err := yaml.UnmarshalStrict([]byte(input), cfg)
+	require.NoError(t, err)
+
+	output, err := yaml.Marshal(cfg)
+	require.NoError(t, err)
+	require.Equal(t, input, string(output))
+}