Implement MarshalYAML for relabel.Config so that we do not generate a regex field if it was not provided in the first place

Signed-off-by: Liam Howe <liam.howe@maersk.com>

model/relabel/relabel.go

@@ -111,6 +111,15 @@ func (c *Config) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	return c.Validate()
 }
 
+// MarshalYAML implements the yaml.Marshaler interface.
+func (c Config) MarshalYAML() (interface{}, error) {
+	// Omit the regex if it is the default regex as it was not provided in the first place.
+	if c.Regex == DefaultRelabelConfig.Regex {
+		c.Regex.Regexp = nil
+	}
+	return c, nil
+}
+
 func (c *Config) Validate() error {
 	if c.Action == "" {
 		return fmt.Errorf("relabel action cannot be empty")

model/relabel/relabel_test.go

@@ -851,3 +851,42 @@ func BenchmarkRelabel(b *testing.B) {
 		})
 	}
 }
+
+func TestConfig_UnmarshalThenMarshal(t *testing.T) {
+	tests := []struct {
+		name      string
+		inputYaml string
+	}{
+		{
+			name: "Values provided",
+			inputYaml: `source_labels: [__meta_kubernetes_pod_annotation_prometheus_io_port]
+separator: ;
+regex: \\d+
+target_label: __meta_kubernetes_pod_container_port_number
+replacement: $1
+action: replace
+`,
+		},
+		{
+			name: "No regex provided",
+			inputYaml: `source_labels: [__meta_kubernetes_pod_annotation_prometheus_io_port]
+separator: ;
+target_label: __meta_kubernetes_pod_container_port_number
+replacement: $1
+action: keepequal
+`,
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			unmarshalled := Config{}
+			err := yaml.Unmarshal([]byte(test.inputYaml), &unmarshalled)
+			require.NoError(t, err)
+
+			marshalled, err := yaml.Marshal(&unmarshalled)
+			require.NoError(t, err)
+
+			require.Equal(t, test.inputYaml, string(marshalled))
+		})
+	}
+}